TECHNICAL FIELD The present invention relates to high strength galvannealed steel sheet and a method of production of the same, more particularly relates to a galvannealed steel sheet having excellent workability and able to be used for various applications such as steel sheet for building materials or for automobiles. BACKGROUND ART As coated steel sheet with a good corrosion resistance, there is galvannealed steel sheet. This galvannealed steel sheet is usually produced by degreasing the steel sheet, then preheating it in a nonoxidizing furnace, cleaning the surface, securing the desired quality by annealing it by reduction in a reducing furnace, dipping it in a hot-dip zinc bath, controlling the amount of deposition, then alloying it. This is characterized by excellent corrosion resistance and coating adhesion etc., so is being widely used for automotive and building material applications etc. In particular, in recent years, in the automobile field, higher strength of coated steel sheet has been considered necessary for securing the function of protecting the passengers against collisions and for reducing weight so as to improve the fuel efficiency. To increase the strength of steel sheet without detracting from the workability, it is effective to add elements like Si or Mn and P, but addition of these elements delays the alloying, so compared with mild steel, a higher temperature and longer time are required for alloying. This higher temperature, longer alloying causes the austenite remaining in the steel sheet to transform to pearlite and reduces the workability, so as a result the effects of these added elements is canceled out. For the alloying of Si-containing high strength steel sheet, Japanese Unexamined Patent Publication (Kokai) No. 5-279829 discloses a method of production realizable even by a continuous hot-dip galvanizing line, but the range of the production conditions is described extremely broadly and this is of little use in actual production. Further, the method of production disclosed in Japanese Unexamined Patent Publication (Kokai) No. 11-131145 causes the formation of retained austenite by holding the sheet at a low temperature after coating, but this invites an increase in the facilities, so detracts from the productivity. DISCLOSURE OF THE INVENTION Therefore, the present invention solves the above problems and provides high strength galvannealed steel sheet excellent in workability and a method of high strengtli galvannealed steel sheet excellent in workability without installing new facilities. The inventors engaged in intensive research on coating high strength steel sheet and as a result discovered that by coating steel to which C, Si, and Mn have been added in certain amounts or more by a continuous galvanizing facility optimized in heat treatment conditions and coating conditions, it is possible to produce high strength galvannealed steel sheet excellent in workability. That is, the gist of the present invention is as follows: (1) High strength galvannealed steel sheet excellent in workability, comprising high strength steel sheet containing, by mass%, C: 0.05 to 0.15%, Si: 0.3 to 2.0%, Mn: 1.0 to 2.8%, P: 0.03% or less, S: 0.02% or less, Al: 0.005 to 0.5%, and N: 0.0060% or less and a balance of Fe and unavoidable impurities, where, when %C, %Si, and %Mn respectively represent the C, Si, and Mn contents, (%Mn)/(%C)>12 and (%Si)/(%C)>4 being satisfied. on the surface of which having a galvannealing layer containing Al: 0.05 to 0.5 mass% and Fe: 5 to 15 mass% and a balance of Zn and unavoidable impurities, said steel sheet satisfying a relationship of tensile strength F (MPa) and elongation L (%) of L>52-0.035xF. (2) A method of production of high strength galvannealed steel sheet excellent in workability comprising finish rolling a slab of a composition of chemical ingredients as set forth in (1) at a temperature of at least an Ara point, cold rolling it by 50 to 85%, then annealing it in a continuous hot dip galvanizing facility in the 700°C to 850°C ferrite and austenite two- phase temperature region, cooling it from its maximum peak temperature to 650°C by an average cooling rate of 0.5 to 10°C/sec, then from 650°C to 500°C by an average cooling rate of 3°C/sec or more, holding it from 500°C to the coating bath for 30 seconds to 240 seconds, then galvanizing coating it so as to form on the surface of said cold rolled steel sheet a hot-dip galvanizing layer, then alloying said steel sheet formed with said galvanizing layer so as to produce a galvannealed steel sheet comprised of said steel sheet formed on its surface with a galvannealing layer, said method of production of high strength galvannealed steel sheet excellent in workability characterized by performing said hot-dip galvanizing in a hot-dip galvanizing bath of a composition of ingredients comprised of an effective Al concentration in the bath of 0.07 to 0.105 wt% and a balance of Zn and unavoidable impurities and performing said alloying at a temperature T (°C) satisfying: 225+25'00x[Al%]52-0.035xF. The reason for limiting the elongation L to [52-0.035xF]% or more is that if L is lower than [52-0.035xF], the sheet will break at the time of deep drawing or other severe working and the workability will otherwise become insufficient. Next, the reasons for limitation of the production conditions will be explained. The object lies in obtaining a microstructure including martensite and retained austenite in an amount of 3 to 20% and achieving both a high strength and good press workability. If the volume ratio of martensite and retained austenite is less than 3%, a high strength will not be obtained. On the other hand, if the volume ratio of martensite and retained austenite exceeds 20%, while the strength will be high, the workability of the steel sheet will deteriorate and the object of the present invention will not be achieved. The slab used for the hot rolling is not particularly limited, but a continuous casting slab or a slab produced by a thin slab caster etc. may be used. A process such as "continuous casting to direct rolling (CC to DR)" performing the hot rolling right after casting is also met with. The finish temperature of the hot rolling has to be made the Ars point or more from the viewpoint of securing the press formability of the steel sheet. The cooling conditions after the hot rolling and the coiling temperature are not particularly limited, but the coiling temperature should not be one giving greater variation in quality at the two ends of the coil and should not be one causing deterioration of the pickling ability due to the increase of the scale thickness, so is made 750°C or less. Further, if bainite or martensite is partially formed, edge cracks will easily occur at the time of cold rolling. In extreme cases, the sheet will even break. Therefore, 550°C or more is preferable. The cold rolling may be performed under the usual conditions. The ferrite is made to be easily work hardened by finely dispersing martensite and retained austenite in it. For the purpose of obtaining the greatest improvement in the workability, the reduction ratio is made 50% or more. On the other hand, cold rolling by a reduction ratio of over 85% requires a massive cold rolling load, so is not realistic. When annealing the sheet by an in-line annealing type continuous hot dip galvanizing facility, the annealing temperature is made the 700°C to 850°C region where the two phases of ferrite and austenite can coexist. If the annealing temperature is less than 700°C, the recrystallization is insufficient and the steel sheet cannot be provided with the necessary press workability. Annealing at a temperatuxe over 850°C results remarkable growth of an Si or Mn oxide layer at the steel strip surface and easily results in coating defects, so is not preferable. Further, in the process of dipping the sheet in the coating bath and cooling it, even if gradually cooling to 650°C, a sufficient volume ratio of ferrite will not be grown, the austenite will transform to martensite in the middle of cooling from 650°C to the coating bath, the martensite will be tempered by the reheating for the later alloying and cementite will be precipitated, so achieving both high strength and good press workability will become difficult. The steel strip is annealed, then cooled in the process of being dipped in the coating bath. The cooling rate in this case is an average 0.5 to 10°C/sec from its maximum peak temperature to 650°C and an average cooling rate of 3°C/sec from 650°C to 500°C. The strip is held from 500°C to the coating bath for 30 seconds to 240 seconds, then dipped in the coating bath. The rate until 650°C is made an average 0.5 to 10°C/sec for the purpose of improving the workability by increasing the volume ratio of ferrite and simultaneously for increasing the C concentration of the austenite so as to lower the free energy produced and make the temperature of the start of the martensite transformation the coating bath temperature or less. To make the average ncooling rate until 650°Cless than 0.5°C.sec it is necessary to lengthen the line of the continuous hot-dip galvanizing facility which results in higher cost, so the average cooling rate until 650°C is made 0.5°C/sec or more. To make the average cooling rate until 650°C less than 0.5°C/sec, it may be considered to lower the maximum peak temperature and anneal the strip at a temperature with a small volume ratio of austenite, but in this case the suitable temperature range becomes narrower than the temperature range allowed in actual operation. If the annealing temperature is low even a bit, no austenite will be formed and the object will not be achieved. On the other hand, if the average cooling rate until 650°C exceeds 10°C/sec, not only will the increase in the volume ratio of ferrite be insufficient, but also there will be little increase in the concentration of C in the austenite, so before the steel strip is dipped in the coating bath, part will transform to martensite. In the heating for the later alloying, the martensite will be tempered and precipitate as cementite, so achievement of both high strength and good workability will become difficult. The average cooling rate from 650°C to 500°C is made 3°C/sec or more so as to avoid the austenite transforming to pearlite in the middle of the cooling. If the cooling rate is less than 3°C/sec, even if annealing at the temperature prescribed in the present invention or cooling to 650°C, pearlite will unavoidably be formed. The upper limit of the average cooling rate is not particularly limited, but cooling steel strip so that the average cooling rate exceeds 20°C/sec is difficult in a dry atmosphere. The reason for holding the sheet from 500°C to the coating bath for 30 seconds to 240 seconds is that if less than 30 seconds, the concentration of C in the austenite becomes insufficient and the concentration of C in the austenite will not reach the level enabling residual presence of austenite at room temperature, while if over 240 seconds, the bainite transformation will proceed too much, the amount of austenite will become smaller, and a sufficient amount of retained austenite will no longer be able to be formed. Further, while holding from 500°C to the coating bath, if once cooling to 450°C or less and holding the sheet for 25 seconds or more, the concentration of C in the austenite will be promoted and a high strength galvannealed coating excellent in workability will be obtained. However, if dipping the sheet into a coating bath at 450°C or less, the coating bath will be cooled and will solidify, so the sheet has to be reheated to a temperature of over 4 50°C, then coated by hot-dip galvanizing. In the production of the galvannealed steel sheet according to the present invention, the hot-dip galvanizing coating bath used is adjusted to an Al concentration of an effective Al concentration C in the bath of 0.07 to 0.105 wt%. Here, the "effective Al concentration in the coating bath" is the value of the Al concentration in the bath minus the Fe concentration in the bath. The reason for limiting the effective Al concentration to 0.07 to 0.105 wt% is that when the effective Al concentration is lower than 0.07%, the Fe-Al-Zn phase serving as an alloying barrier at the initial part of the coating is insufficiently formed and the brittle F phase is formed thicker at the coating-steel sheet interface at the time of coating, so only galvannealed steel sheet inferior in coating adhesion at the time of working can be obtained. On the other hand, when the effective Al concentration is higher than 0.1051, high temperature, long period alloying becomes necessary and the austenite which had remained in the steel transforms to pearlite, so achievement of both high strength and workability becomes difficult. Further, in the present invention, the alloying temperature at the time of alloying is made a temperature T(°C) satisfying 225+2500x[Al%]12 and (%Si) / (%C) > 4 being satisfied, on the surface of which having a galvannealed layer containing Al: 0.05 to 0.5 mass% and Fe : 5 to 15 mass% and a balance of Zn and unavoidable impurities, said steel sheet satisfying a relationship of tensile strength F (Mpa) and elongation L (%) of L >52-0.035xF. 2. A method of production of high strength galvannealed steel sheet comprising (a) finish rolling a slab of a composition of chemical ingredients as set forth in claim 1 at a temperature of at least an Ar3 point, (b) cold rolling it by 50 to 85%, (c) then annealing it in a continuous hot dip galvanizing facility in the 700°C to 850°C ferrite and austenite two-phase temperature till 650°C by an average cooling rate of 0.5 to l0°C/sec, then from 650°C to 500°C by an average cooling rate of 3°C/sec or more, (d) holding it from 500°C to the coating bath for 30 seconds to 240 seconds, (e) then hot-dip galvanizing it so as to form a. hot-dip galvanizing layer on the surface of said cold rolled steel sheet, (f) then alloying said steel sheet formed with said hot-dip galvanizing layer so as to produce a galvannealed steel sheet comprising said steel sheet having a galvannealed layer formed on its surface, wherein said hot-dip galvanizing is performed in a hot-dip galvanizing bath of a composition of ingredients comprised of an effective Al concentration in the bath of 0.07 to 0.105 mass% and a balance of Zn and unavoidable impurities and said alloying is performed at a temperature T (°C) satisfying: 225+2500x[Al%]