Python 3 compatibility fixes.

- Don't use ambiguous imports in a package, use relative imports.
- TestCase.assertItemsEqual() was removed in Python 3.2 (renamed to assertCountEqual()).
Run tox / travis-ci against Python 3.4.
Removed duplicate Python environments from Travis, tweak test runner so we don't
get any warnings.
Merge pull request #13 from davidwtbuxton/py3-compatibility

Py3 compatibility