Add bugsnag support

Hi, I added support for Bugsnag webhook :)

> [Available Plugins - Bugsnag](https://bugsnag.com/docs/notification-plugins)

![ss 2014-10-10 at 23 17 52](https://cloud.githubusercontent.com/assets/1041857/4594216/904b31ca-508d-11e4-84a3-4859918289d4.png)
In this case, a string literal is unnecessary. Instead of:

``` haml
#{payload.user.name} commented on
```