Implement ModeratorInvite. Fix #132
Fix test_get_hidden StopIteration Error

A submission is hidden at the beginning of test_get_hidden to avoid
eventually running out of hidden Things.
Refactor handling of moderator invites.

To be consistent with reddit I decided making a ModeratorInvite subclass of
Messge is not the best idea. After discusison on IRC the best course of action
was to simply try to accept the invite.