User Check Out

Closes #51

- Ensures consistent user state by "checking out" the user from a venue on startup, shutdown and authorization events.
- Adds checkoutWithSuccess:andFailure: to VBUser
User source property + listener count

Parent  #56
Closes  #55
Relates #57

- Adds `source` property to `VBUser`
- Adds `listenerCountWithSuccess:andFailure:` to `VBUser`
- Adds `VBUserEventSourceChanged` event, triggered on source change
- Modifies `VBInputSourceManager` to observe `VBUserEventSourceChanged` and change input
- Moves `sourceUser` to private property of `VBInputSourceManager`
- Adds test method for user source count
- Nils source on user check out
Merge pull request #57 from azoff/55-user-count

User source property + listener count