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)







Hmm. I sort of dislike that this creates a discrepancy between how instructions for setting pagerbot up for slack and hipchat.

Unless you have any strong opinions here, I would move the contents of this paragraph into `bot.html`?