Closes #341: (iOS) allow callers of `FB.login` to detect cancel.

Per Facebook JS SDK docs, cancel should call the success callback with a null
`response.authResponse`.

Partially closes #327: (iOS) call `[FBSession.activeSession
handleDidBecomeActive]` in response to `applicationDidBecomeActive` events (per
https://developers.facebook.com/docs/tutorials/ios-sdk-tutorial/authenticate/)