Feature/plans as models

This request is to facilitate the conversation happening in issue [pydanny#22](https://github.com/pydanny/dj-stripe/issues/22#issuecomment-35106537).

*This isn't the prettiest request, as a bunch of the diff is PEP8 stuff*

Two relevant components of this request:
- The [new Plan model](https://github.com/briandant/dj-stripe/compare/feature;plans-as-models?expand=1#diff-09f7e19fc1e307e1253732bfa4748b2aR893)
- [The associated Admin](https://github.com/briandant/dj-stripe/compare/feature;plans-as-models?expand=1#diff-d2b93f9be925732ca424463a9c6461beR296)