Add BugHerd Integration

https://github.com/bugsnag/bugsnag-notification-plugins/issues/23

Stacktraces look a lil' cramped, if you have any ideas, feel free to express!

Supports:
- [x] Setting priority
- [x] Adding tags
- [x] Specifying a status/list

Screenshot:
![Screenshot](http://cl.ly/image/0G1f2l0U3X1S/Image%202014-07-19%20at%205.53.40%20pm.png)