HipChat custom server support

HipChat allows users to host their own custom instance. This I believe adds support for that based on the HipChat Ruby bindings API.

Caveat: I don't know Ruby at all. Did this with a few minutes of Googling and looking at hipchat-rb docs. Not sure if it even works. I don't have an environment to test GitLab.
Use 2 (not 4) spaces for indentation.