Add test coverage

[pydanny]

Using 4242424242424242 as a test card, there should be comprehensive test coverage of this project. Otherwise it is hard to recommend to clients and customers.

[skoczen]

Completely agree. I'm a functional-testing believer, so the dependency to getting this working is another project (https://github.com/GoodCloud/django-functional-tests) fully abstracted, and writing tests up.

If you'd like to write tests before that, they're quite welcome!

[leetrout]

I did start on unit tests for the mixins but then I got sidetracked rewriting the stripe lib tests and never came back. I might have time to finish those this weekend.

I second skoczen and if someone else has the time they're more than welcome.