The present invention relates to a method for producing the binder resin. TECHNICAL FIELD The present invention relates to a binder resin for a toner and a method for producing the same. Furthermore, the invention relates to a toner for electrophotography using the aforementioned resin. BACKGROUND ART With recent demands of copying at high speeds and fixing at low energies and the like, toners with fixing properties at a temperature as low as possible have been in demand. In response to the demand, there has been proposed a method for enhancing fluidity of a binder resin for a toner by increasing the ratio of the low molecular weight portion . However, this method has a problem of causing the deterioration in offset resistance. Therefore, there has been reported a method for balancing the fixing properties and offset resistance by introducing a ge! component into a binder resin for a toner. A toner used for copiers or printers gives off an odor in printing through the melting process, if a volatile solvent used in the production of toners, an unreacted raw material or the like remains in the toner. In recent years, information appliances have been rapidly come into use and a plurality of printers have been installed in offices. Therefore, development of a toner with a small amount of the remaining volatile component has been demanded from the market. Until now, as a method for removing a volatile component remained in a resin which does not contain a gel component, JP2000-143721A (Patent Document 1) has been proposed. However, in case a volatile component is removed from the resin containing a gel component using this method, there is a possibility that toners might not be stably produced due to the Weissnberg effect. Namely, a toner containing a gel component and having a small amount of a volatile component has not been discovered yet. [Patent Document 1] JP2000-143721A DISCLOSURE OF THE INVENTION Accordingly, the present invention provides a binder resin for a toner which contains a styrene-acrylic resin having a gel component and a small amount of a volatile component, and a method for producing the same. Furthermore, the invention provides a toner using the binder resin for a toner. The present inventors have conducted an extensive study and completed the present invention. Thus, the present invention is: a binder resin (A) for a toner which comprises a styrene-acrylic resin having a structure derived from a carboxyl group and a structure derived from a glycidyl group, wherein the content (A|S) of a gel component is 1 mass % ≤ Ais ≤50 mass % and the content (AVo) of a volatile component in the resin is AVo ≤ 200 ppm; a toner for electrophotography compriseing the binder resin (A) for a toner as described in (1); and a method for producing the binder resin (A) for a toner as described in (1) comprising the following processes (I), (II) and (III) in this order: Process (I): a process comprising melt-mixing a carboxyl group-containing vinyl resin (B) and an epoxy group-containing vinyl resin (C) at a temperature (TR) satisfying 120°C ≤ TR ≤230°C in a twin screw extruder for the reaction; Process (II): a process comprising introducing water into the twin screw extruder, and mixing water with the resin composition obtained in the process (I) under the conditions satisfying a pressure (pex) of 1 MPa ≤ PEX ≤ 2.7 MPa and a temperature (TM) of 120°C≤TM≤ 230°C;and Process (III): a process comprising reducing the pressure inside the twin screw extruder for removing water and the volatile component. EFFECT OF THE INVENTION According to the present invention, it is possible to obtain a binder resin for a toner having a small amount of a volatile component even if the resin contains a gel component. Thus, a toner using the binder resin for a toner of the present invention is capable of reducing an unpleasant odor or the like during use . BEST MODE FOR CARRYING OUT THE INVENTION The binder resin (A) for a toner of the present invention is a styrene-acrylic resin having a structure derived from a carboxyl group and a structure derived from a glycidyl group. Furthermore, the content (A|S) of the gel component is 0.1 mass % ≤ A|S ≤ 50 mass %, preferably 1 mass % ≤A|S ≤20 mass %, and more preferably 1 mass % ≤ A|S ≤ 16 mass %. By having the content of the gel component within the above range, both good offset resistance and fixing properties can be exhibited. In the present invention, a gel component refers to a component that is not dissolved in ethyl acetate, in a resin. The content (Avo) of the volatile component of the binder resin (A) for a toner according to the present invention is AVo ≤ 200 ppm and more preferably AVo ≤100 ppm. By having AVo within the above range, a toner obtained by using the binder resin (A) for a toner does not cause any odor in printing. The binder resin (A) for a toner according to the present invention is not particularly restricted as far as it exhibits sufficient performance as a toner to be described later. However, it is preferably produced by the reaction of a carboxyl group-containing vinyl resin (B) and an epoxy group-containing vinyl resin (C). A method for producing the carboxyl group-containing vinyl resin (B) is not particularly restricted, but it is preferably obtained by copolymerizing a vinyl monomer and a vinyl monomer having a carboxyl group. Examples of the vinyl monomers include styrene based compounds such as styrene, p-methylstyrene, a-methylstyrene, vinyl toluene and the like; acrylic esters such as methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, octyl acrylate, cyclohexyl acrylate, stearyl acrylate, benzyl acrylate, furfuryl acrylate, hydroxyethyl acrylate, hydroxybutyl acrylate, dimethylaminomethyl acrylate, dimethylaminoethyl acrylate and the like; methacrylic esters such as methyl rnethacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, octyl methacrylate, cyclohexyl methacrylate, stearyl methacrylate, benzyl methacrylate, furfuryl methacrylate, hydroxyethyl methacrylate, hydroxybutyl methacrylate, dimethylaminomethyl methacrylate, dimethylaminoethyl methacrylate and the like; diesters of an unsaturated dibasic acid such as dimethyl fumarate, dibutyl fumarate, dioctyl fumarate, dimethyl maleate, dibutyl maleate, dioctyl maleate and the like; acrylonitrile; methacrylonitrile; and amides such as acrylamide, methacrylamide, N-substituted acrylamide, N-substituted methacrylamide and the like. Among these, particularly preferable examples of the vinyl monomers include styrene based compounds, acrylic esters, methacrylic esters, dialkyl fumarate esters, acrylonitrile, acrylamide, methacrylamide and the like. Examples of the vinyl monomers having a carboxyl group preferably include acrylic acid, methacrylic acid, maleic anhydride, maleic acid, fumaric acid, cinnamic acid, and mono esters of an unsaturated dibasic acid such as methyl fumarate, propyl fumarate, butyl fumarate, octyl fumarate, methyl maleate, ethyl maleate, propyl maleate, butyl maleate, octyl maleate and the like. Particularly preferable are acrylic acid, methacrylic acid, furnaric acid, methyl fumarate, ethyl fumarate, propyl fumarate, butyl fumarate and octyl fumarate. A method for producing an epoxy group-containing vinyl resin (C) is not particularly restricted, but it is preferably obtained by copolymerizing the aforementioned vinyl monomer and a vinyl monomer having an epoxy group. Examples of the vinyl monomers having an epoxy group include glycidyl acrylate, p-methylglycidyl acrylate, glycidyl methacrylate, (ß-methylglycidyl methacrylate and the like. Among these, preferable are glycidyl methacrylate and p-methylglycidyl methacrylate. The glass transition temperature (TgB) of the carboxyl group-containing vinyl resin (B) is preferably 40°C ≤TgB :≤ 70°C, and more preferably 50°C≤TgB≤ 60°C. The glass transition temperature in the present invention is measured in accordance with JIS K-7121. The glass transition temperature within the above range is preferable because both the offset resistance and fixing properties can be exhibited. An acid value of the carboxyl group-containing vinyl resin (B) is preferably from 1 KOHmg/g to 30 KOHmg/g and more preferably from 5 KOHmg/g to 30 KOHmg/g. The acid value within the above value is preferable because the reaction with the epoxy group-containing vinyl resin (C) properly proceeds so that both good offset resistance and fixing properties can be exhibited. The acid value in the present invention refers to mg of potassium hydroxide necessary to neutralize 1 g of the resin, and it is a value measured in accordance with JIS K-5601-2-1. The weight-average molecular weight (cmw) of the epoxy group-containing vinyl resin (C) is preferably 10,000 ≤cmw ≤ 100,000, more preferably 15,000 ≤CMw ≤ 85,000, and particularly preferably 25,000 ≤cmw ≤ 75,000. The weight-average molecular weight (cmw) within the above range is preferable because polymerization and gelation properly takes place by the reaction with the carboxyl group-containing vinyl resin (B) so that the mechanical strength and the viscosity are balanced and both excellent durability and fixing properties can be exhibited. The epoxy equivalent (CEp) of the epoxy group-containing vinyl resin (C) is preferably 1,000 g/Eq≤ CEP ≤ 20,000 g/Eq, more preferably 1,000 g/Eq ≤ CEP ≤15,000 g/Eq, and particularly preferably 1,000 g/Eq≤cep ≤ 10,000 g/Eq. The epoxy equivalent (CEp) within the above range is preferable because the reaction with the carboxyl group-containing vinyl resin (B) properly takes place so that good offset resistance and fixing properties can be exhibited. The epoxy equivalent in the present invention refers to the mass (g) of an epoxy resin per 1 g equivalent of an epoxy group. The epoxy equivalent can be obtained by a known method, for example, the HCI-dioxane method as described in pp. 161 to 164 of [Epoxy Resin] written by Hiroshi Kakiuchi (Shokodo Co., Ltd.). The binder resin (A) for a toner of the present invention may includes an epoxy resin, a polyester resin, a xylene resin, polyolefin and the like. The binder resin (A) for a toner according to the present invention can be preferably produced by the following process. Process (I): A process comprising melt-mixing a carboxyl group-containing vinyl resin (B) and an epoxy group-containing vinyl resin (C) at a temperature (TR) satisfying 120°C