Add tests

I think all code should have tests.

The tests shouldn't depend on the Authorize.NET servers so [mock][] should probably be used to mock calls to `request.post` and other network calls as needed.

I may turn this into a pull request if I decide to start writing some tests.

Goals of this PR:

* [x] Add basic test suite allowing `python setup.py test` to work
* [x] Add a few example tests
* [x] Add tox file for supported Python/Django versions
* [x] Add Travis and Coveralls support