FogBugz Integration

https://github.com/bugsnag/bugsnag-notification-plugins/issues/39

This integration makes up to 3 requests (based on the mode of authentication).

If the user chooses to enter their email & password, we need to send a request (firstly) to get the token.
Then we send the 'case creation' request.
And then we have to logoff the token (not sure if it's needed, but it was documented, so it's possible you only have a limited amount of 'logged in' tokens/sessions per user).

The the user provides a pre-generated token, we only need to make one request, and that is to create the case. And then we leave the token, we don't log off it, incase we need to use it for later.

The FogBugz API is actually pretty well documented, but I would still love to find a nice way of creating the case with an HTML rendered body. *That's on the TODO, for later*

**Features**
 - [x] Applying Tags
 - [x] Assigning a user
 - [x] Applying an Area
 - [x] Setting a priority
 - [x] Multi-login options (email + password / token)

**TODO**
 - [ ] Hande re-opening when re-occur (like GitHub)
 - [ ] HTML rendered body (currently plain text - with `code` tag)

![Screenshot](http://cl.ly/image/2v043x2V2i3F/Image%202014-07-29%20at%2011.07.27%20pm.png)

I'm going to have another look over the code tomorrow, as a sort of 'self review', as it's late, and I could not possibly perform one now :smile: 
I did not know you could do that, very cool