Additional functionality and Python 3 support

Sorry for such a big changeset, found your lib useful but needed other things mocked in the response and got a little carried away.

- Support Python 2.6, 2.7, 3.2, 3.3
- Add all_requests decorator for non-conditional mocking
- If mock_method returns dict, will map keys to Response object
- Add response method that will build out a mostly populated Response object including cookies
- Add travis-ci testing config (https://travis-ci.org/gmr/httmock points to my fork)