Added v2 flavor of the HipChat notification API

Hello,

I noticed that the current hipchat.rb service uses an outdated version of their API (v1 instead of v2). The main difference that I ran into is I can't use a per-room auth key using the v1 API, instead I'd need to get one from the organization's group admin.

This pull request adjusts the existing HipChat service to use the v2 API calls. Most of the differences are with how the `http` object is set up and which endpoint is getting called.

Thanks!