Careful with unsigned or invalid messages in fedmsg-irc.

All message consumers in fedmsg-land should inherit from ``fedmsg.consumers.FedmsgConsumer``, and if they do, they have their messages validated for them beforehand.  The IRCConsumer is an exception.  It explicitly disables this behavior because I wanted to use it for debugging way back in the beginning.  Now that we're all grown up, it should probably behave a little more safely.

This code makes it so that it touches the contents of a message as little as possible if it is unsigned or has an invalid signature.  It used to pass it off to ``fedmsg.meta.*``, but there's a lot of code there where something could go wrong.  Now, we just echo the message's topic to channel with a note that it is invalid and move on.