VBVenue Model

Closes #19

- Implements VBVenue Model Subclass
- Corrects PFObject subclassing to conform with PFSubclassing protocol
- Adds dynamic fields for VBUser and VBVenue
- Registers subclasses with parse at app startup
- Adds asynchronous methods to check in as a user, and fetch checked in users as a venue (one to many)
- Adds test for current user
- Adds test for the anonymous user
- Adds user count method