Add HipChat2 messenger for V2 of the HC API

Hey folks,

NOTE: I don't know if this is something you're interested in, but I needed it
to work with our HipChat setup (we've only got access to V2 API tokens).

 I've implemented a super barebones version of the HipChat V2 API using the
 https://github.com/tbruyelle/hipchat-go library. I've tested it on our own
 setup successfully, but I wasn't sure what the standard place to put consumer
 documentation was. I'd be happy to add something if you can point me in the
 right direction.