User source property + listener count

Parent  #56
Closes  #55
Relates #37

- 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