Mock isn't supported on python 3.4

It is included in unittest starting with 3.3.
Change tests to try importing unittest.mock first, then falling back to mock.
Change setup.py to only require mock for tests on python < 3.3.
All tests pass on 3.4 with this change.
Update test requirements
Add new classifier for Python 3.4
Only import mock in tests/utils.py
Update references to mock in api tests
Update references to mock in GitHub tests
Update references to mock in Issues tests
Update references to mock in Organization tests
Update references to mock in PR tests
Update references to mock in Repo tests
Update references to mock in Iterator tests
Update references to mock in tests/unit/
Add Python 3.4 to Travis config
Merge branch 'pr/227' into develop
Fix broken mock references due to merge