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)







@macobo - Only messages starting with configatron.bot.name are sent to the webhook. This is mentioned in the initial JSON document where we specify the webhook details. 

https://github.com/rohitpaulk/pagerbot/blob/hipchat/lib/pagerbot/hipchat_adapter.rb#L39