Add Hipchat Support

fixes #3.

This is implemented as a [HipChat addon](https://www.hipchat.com/docs/apiv2/addons). 

No copy-paste configuration is needed, all that is handled by a HipChat webhook. Here's a [live demo](https://pagerbothipchat.herokuapp.com/) of the admin app.

When it's time to deploy, one would set `DEPLOYED` to `true` as usual and then go to "https://hipchat.com/admin/addons" and click on Build and install your own integration. After that, he'd paste the URL of the app in the modal, and the configuration will be done automatically. Hipchat sends in the OAuth ID and Secret to a specified URL, and the app manages the rest. :)

![hipchat](https://cloud.githubusercontent.com/assets/3893573/4841479/1781ddd2-601b-11e4-8656-2dcbe327ac7b.png)







Yep. 

> When the HipChat administrator chooses to uninstall your add-on, you will receive a HTTP DELETE request, but this time with the OAuth ID appended to the path, so if your installable URL is https://example.com/installable and the OAuth ID value for the installation was "12345", the URL for uninstallation will be https://example.com/installable/12345. Whether your application returns a 200 response or not, the uninstallation will be allowed to continue.