Implements PubSub Counters for TableViewCells

Closes #80

- Updates VBPubSub to support the notion of venues and users
- Deletes some facebook code to get rid of warnings
- Adds pubsub methods for venues
- Removes user query and count methods from VBVenue
- Updates VBUserDataSource to use Firebase as the cell content source
- Adds cached query getter for FoursquareObjects
- Updates VBUserTableViewCell to use Firebase as the counter source
- Publish check[in|out] events in VBUser methods. Re: #90
- Updates checkin method to checkout first
- Clears out user pubsub channel on checkout. Closes #90
- Post locally when not logged in
- Adjusts user data source to make multiple calls to the controller
- Adds methods to subscribe, publish, and redact source listeners
- Updates VBUser to publish listening and unlistening to sources
- Removes source on checkout
- Send to venues screen if authorized
- Adds specific unsubscribe methods to PubSub; fixes orphaned handler bugs