Some cleanup and a couple of new features

[moorchegue]

This might be a bit messy to reviewb, but basically there are just a few changes:

1. Constructor takes requests_kwargs and passes them to requests calls.
2. Constructor takes response_wrapper which might be just a json.loads or something more fun to play with request object, response headers, etc.
3. PaginatableResponse wrapper implemented to suck everything out of paginatable endpoint e.g. api.github.com/users. Examples provided.