Refactored the way tweepy does parsing to make it more customizable by developers.

Refactored the way tweepy does parsing to make it more customizable by developers.

All parsing of the response payload is now handled by a Parser class defined in
tweepy/parsers.py
The default parser used is ModelParser which parses a JSON payload into a model instance.

Developers may define and use their own custom parsers by extending the Parser class.
To use the custom parser:

    api = API(parser=MyParser())