Add support for Django 1.7

Remove the call to `get_user_model` from the `models.py` file, and
instead import and call it directly in `backends.py`.

In addition, added a test suite to show this works with 1.5, 1.6, and
1.7 and all the wiring to have it run through travis-ci.
Merge pull request #3 from pipermerriam/piper/setup-test-suite-and-ci

Django 1.7 support plus testing harness