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
in https://github.com/sensu/sensu-community-plugins/pull/793/files#diff-76e998229a93b6780ab1db9b03f3bb95R247
you use interpolation, but here concatenation. Can you please use interpolation everywhere as it is better practice across ruby community