Add IRC notifier, update README and tests accordingly

First of all, thanks for a great gem! We use it often and have never had issues.

We have a company IRC server, so I wanted to be able to send exception messages there. I'm not sure if this is within the scope of the project. If it's not, no big deal; we'll stick to our fork.

The notifier sends messages to an IRC channel using carrier pigeon. It can be configured with several options, which are explained in the README.

Thanks!

(I closed an almost duplicate request a little while ago because I was stupid and neglected to include Gemfile.lock in the commit. Sorry about that!)