Add get ident method to base throttle class

This pull has been moved from: https://github.com/tomchristie/django-rest-framework/pull/1222#issuecomment-30031714

This pull adds a get_ident method to the BaseThrottle class. This function is used to determine the identity of a request and will use `X_FORWARDED_FOR` and `REMOTE_ADDR` in that order to determine the identity of the machine making the request. It also allows a developer to configure the number of application proxies and parses `HTTP_X_FORWARDED_FOR` accordingly.