Additional functionality and Python 3 support

[gmr]

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)