Add get_ident method to BaseThrottle class

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.

* Tries to use X_FORWARDED_FOR first
* Falls back to REMOTE_ADDR