Better Foursquare Integration

Closes #62

- Modifies the Foursquare library to allow capture of "Cancel" event
- Adds comment in app delegate for testing auth flow
- Modifies checkin controller to check auth status on load
- Modifies checkin controller to return to source controller on auth fail
Merge pull request #67 from azoff/62-foursquare-checkout

Better Foursquare Integration