Support Check-ins

- [x] Add movie and episode check-in methods
- [x] Add delete active check-in method
- [x] Support check-in response
- [x] ~~Add custom `ErrorHandler` to retrofit to support check-in failure (e.g. add wait time to thrown exception)~~ Provide `CheckinError` type to parse error response body by calling `RetrofitError.getBodyAs()`