Pusher disconnect

@adamshone As part of creating the Android example I wanted to be able to connect/disconnect. This pull request does a few things but the main one is the disconnect functionality which I'd like you to review.

See: https://github.com/leggetter/pusher-android-example/blob/master/src/com/pusher/android/example/MainActivity.java#L37

But this may have exposed a bug. If you Connect, Disconnect and then Connect it crashes. We can deal with this elsewhere but wanted to give you a heads-up.