Add TVDB support. Fixes #31

- Upgrade to the latest Android Gradle plugin.
- Add an Interface for the TVDB service.
- Add a basic test cases for testing the TVDB integration.
- Use TVDB client as a fallback if TMDB service fails to fetch details of a TV show or.
- Update README to include where to add TVDB api key

@jerrellmardis @JakeWharton @MizzleDK I finally found some time to work on this. I would appreciate a review. In particular which fields we're interested with the TVDB API.