Fixes #297 New unit tests based on responses

This is ready for review. I'm going to stop work on it until I have feedback.

1. Added a set of unit tests for creating requests using dropbox/responses so no external dependencies
2. Moved the endpoint integration tests to test_endpoints.py
3. Skipped the tests that have been failing. Maybe the OAuth token is not good anymore (a good reason to update them to use a stub or Betamax) Link to an example of the tests failing (82 of them!): https://travis-ci.org/ryanmcgrath/twython/builds/16699968
4. Made updates to the new unit tests to get them to run on 2.6, 2.7, and 3.3
5. Removed the unit test for cursor - it needs to be rewritten. I'll work on that next.

The test coverage dropped to 55%, but most of the tests that I skipped were sanity tests - they did not test anything specific, but made sure no exceptions were thrown.