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!