Callback for a user signing out.

Hi!

First of all, thank you for a very nice service and API! :-) During initial testing in my application I sometimes managed to click sign out (which then triggered signOut() and immediately reflected this in the user interface, as the examples), without completely being signed out. This resulted in an inconsistency between the back end and the user interface. Adding a callback for when the GameHelper successfully manages to sign out an user fixed this (in my case). Examples have been updated, but not very thoroughly  tested.

There could be a very fundamental reason for why this was not implemented in the first place, in which case just ignore this PR. :-)