Better support of async by handling deferred #129

As a plugin developer
I want to return deferred objects from plugin methods
So that I can easily unit test such methods

I think what is needed is for plugins.core.Registry.process to handle twisted.internet.defer.Deferred objects, rather than (or in addition too) catching ResponseNotReady. It should break if first_responder and continue otherwise. (I think this is a bug in the current implementation, ResponseNotReady breaks without considering first_responder).

Also a callback could be added to the Deferred to write the output to channel and update last_message (this currently happens in Client.privmsg though, not sure if you want to filter Deferreds out of the returned responses there though)