Change admin command checks to a decorator

[richteer]

Was considering changing the typically same boilerplate of

def foo(self, msg):
    if self.xmpp.isadmin(msg['from']):
        pass
    else:
        self.xmpp.reply(msg, "Berating message goes here")
        return

to something a little prettier, like:

@admin
def foo(self, msg):
    pass

I think this may coincide with the proposed API change a bit better too, to make the module writing cleaner, and more concise