FIELD OF THE INVENTION The present invention relates to the fields of vehicle theft detection. The present invention specifically relates to a method and system for automatically detecting, notifying and tracking vehicle theft. BACKGROUND OF THE INVENTION In general, vehicle thefts have a tendency to increase, which is becoming an object of public concern. In recent years, various vehicles such as automobile or auto-bicycle, are generally equipped with anti-theft dedicated devices conceived for preventing vehicle theft by an unauthorized person. For example, these vehicle antitheft devices are designed for giving an alarm to the user about the vehicle theft. Such antitheft devices often employ a portable transponder incorporated in an exclusive ignition key that includes a specific identification code on an electronic chip. On the vehicle side, the antitheft device also has a specific identification code. The engine of the vehicle can start or stop only when it is electronically verified that both identification codes match. In this conventional approach, the vehicle theft is prevented from being started by mechanical and/or electrical means when it is attempted to start or move by using a wrong ignition key. The automatic vehicle theft detection is achieved by reading the identification code previously stored in the key when the key is inserted, comparing it with a reference identification code stored in the vehicle side, and generating an enable signal to start the engine only when there is a match between both identification codes. Such existing way of automatic theft detection in vehicles is highly dependent on immobilizer to provide the theft input. Presently, many of the vehicles are not equipped with the immobilizers. In the absence of Immobilizer, even if the vehicle is mounted with the telematics unit, it may not help in automatic vehicle theft detection at it is totally dependent on the immobilizers. In such vehicles, automatic theft detection is not possible, so that the user has to personally ensure the position of the vehicle. If the owner observes that the vehicle is not at the parked location, then the tracking of stolen vehicle is initiated through the telematics system fitted in the vehicle. There are several failure modes for this manual theft detection. For example, if the owner is out of station or gone out of sight of the vehicle for long periods of time, then there is a possibility that the vehicle might be stolen, dismantled or tampered with. With respect to the conventional methods, it is needed that expensive immobilizer should be Interfaced to the telematic units for achieving automatic vehicle theft detection. Such interfacing of Immobilizer with the vehicle telematics unit (TU) requires a dedicated vehicle bus, and also the micro controller on the TU needs additional vehicle bus transceiver interface. Further, it is necessary to have an additional control device to trigger the telematics unit about the vehicle theft, which furuher increases the vehicle cost. Moreover, it does not provide any automatic vehicle theft detection or indication when the vehicle is stolen by towing the vehicle, i.e. without switching on the vehicle engine. Hence, it is desirable to provide a method and system for automatically detecting, notifying and tracking vehicle theft, which is capable of overcoming the aforementioned drawbacks. OBJECT OF THE INVENTION An object of the present invention is to provide a method for automatically detecting, notifying and tracking vehicle theft, which is capable of preventing vehicle theft in a highly cost effective manner and without the need for immobilizer. Another object of the present invention is to provide a system for automatically detecting, notifying and tracking vehicle theft, which achieves automatic theft detection without requiring any modification in telematics system. SUMMARY OF THE INVENTION According to one aspect, the present invention, which achieves the objectives, relates to a method for automatically detecting, notifying and tracking vehicle theft comprising locking a vehicle and a telematics unit upon receiving a lock request from a user communication device while an ignition is turned off. An acknowledgement signal along with vehicle position infomnation can be transmitted to the user communication device. An ignition status and vehicle position can be periodically determined and compared with respect to the ignition status and the vehicle position during locking condition. Theft indication along with a present vehicle position can be notified to the user communication device, if any change is detected in the ignition status and the vehicle position. The vehicle position information can be tracked and updated to the user communication device either automatically or upon receiving a query request from the user communication device. Thus, the method enables the user to easily trace the stolen vehicle, which prevents vehicle theft in a highly cost effective manner. Furthermore, the locking of the vehicle and the telematics unit comprises authenticating the user communication device after receiving the lock request. The vehicle and the telematics unit can be locked only if the lock request is received from the authenticated user communication device. The ignition status and the vehicle position are recorded during the locking condition, using a GPS unit. The vehicle can be automatically locked even when it exceeds a desired duration of ignition off condition. The acltnowiedgement signal can be fonvarded for indicating tlie locking or unlocking condition of the vehicle to the user communication device. For normal vehicle operation, an unlock request is transmitted from the authenticated user communication device to activate the telematics unit, if the vehicle is in locking condition. The lock request, the unlock request, the acknowledgement signal and the vehicle position infomnation are communicated between the user communication device and the vehicle communication device of the telematics unit. According to another aspect, the present invention, which achieves the objectives, relates to a system for automatically detecting, notifying and tracking vehicle theft comprising a GPS unit in communication with a GPS satellite for acquiring vehicle position information. A vehicle communication device is associated with a user communication device for communicating command signal, acknowledgement signal and the vehicle position infomiation. A control means is interfaced with an ignition system, the GPS unit and the vehicle communication device for locking/unlocking a vehicle and a telematics unit based on the command signal received from the vehicle communication device. The control means periodically detemiines an ignition status and the vehicle position information during locking condition. If any change is occurred in the ignition status and the vehicle position infomiation with respect to the ignition status and the vehicle position information at the locking condition, the control means initiates the vehicle communication device to notify theft indication along with a present vehicle position to the user communication device, which achieves automatic theft detection and notification without the need for immobilizer and without requiring any additional modification in the vehicle telematics system. Moreover, the control means is communicated with the GPS unit to track and update the vehicle position infomiation to the user communication device either automatically or upon receiving a query request from the user communication device. The control means is connected v\nth an external memory for storing the command signal, the status signal and the vehicle location information. The control means is configured with authentication information of the user communication device for authenticating the user communication device. The command signal comprises lock request, unlock request and vehicle location query request. The command signal, the acknowledgement signal and the vehicle position information are carried out as SMS message by means of cellular communication. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be discussed in greater detail with reference to the accompanying Figures. FIG. 1 illustrates a schematic view of a system for automatic detection, notification and tracking of vehicle theft, in accordance with an exemplary embodiment of the present invention; FIG. 2 illustrates a sequence diagram depicting automatic detection, notification and tracking of vehicle theft, in accordance with an exemplary embodiment of the present invention; and FIG. 3 illustrates a flow chart of a method for automatically detecting, notifying and tracking vehicle theft, in accordance with an exemplary embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1, a schematic view of a system for automatic detection, notification and tracking of vehicle theft is illustrated, in accordance with an exemplary embodiment of the present invention. The system comprises a telematics unit (100) that is installed in the vehicle and includes a host controller (20), a global positioning system (GPS) unit (10) and a vehicle communication device (30). The telematics unit (100) is a wireless communication unit for providing diverse applications in automotive industry, i.e. for detecting the vehicle theft and tracking the stolen vehicle based on the inputs fed into the telematics unit (100). The GPS unit (10) is in communication with a GPS satellite (200) for receiving vehicle position information including latitude, longitude, etc. The vehicle communication device (30) Is communicated vHth a user communication device (40) through a cellular tower (300) for communicating command signal, status signal and the location information of the vehicle. The communication devices (30, 40) include, but are not limited to GSM or CDMA or GPRS phone. The user communication device (40) directly communicates with the telematics unit (100) in the vehicle through a communication diannels like GSM, CDMA, GPRS, etc. The GPS unit (10) and the vehicle communication device (30) are interfaced to the host controller (20) via universal asynchronous receiver/transmitter (UART) interface or serial communications interface (SCI). The host controller (20) is associated with a vehicle battery and an ignition control system for receiving battery power status and ignition status of the vehicle. The host controller (20) can also be connected with an external memory for storing data the command signal, the status signal and the position information of the vehicle. The host controller (20) is configured with authentication infomiation of the user communication device (40) for avoiding accessing of the telematics unit (100) in the vehicle by other unauthorized communication devices. The authentication information of the user communication device (40) can normally be done at the service station during the time of purchase or regular service. In case, the user losses the pre-registered communication device (40), then the user can again programmed, to the telematics unit (100), with authentication infonnation of a new user communication device as authorized communication device at the service station. This system requires only ignition and battery inputs from the vehicle for automatic detection, notification and tracl