Add Python 3 support

This PR adds support for Python 3.4 by switching to relative import paths and using a version check to deal with string types between this and 2.x versions.

I've also added 3.4 to the travis build matrix, added the Trove classifiers in setup.py and updated the README to reflect these changes.

There are two failing tests, `test_create_email_bad_subject` & `test_create_email_bad_html`, but they are also failing on the upstream master.