Added Hipchat extension

This extension was originally written by @steveberryman but I'm publishing it here with his permission. (With minor modifications.)

The original is here: https://gist.github.com/steveberryman/c57dd6595429e86d6a12
I think it's bad form to depend on an external gem to require the libraries required to run the code.

These are nitpicks and I don't think they should prevent the PR being accepted.