Add Bugzilla integration

I'm not sure what would happen if the user typed wrong, for example, username in the "assigned-to" field. If that happens, Bugzilla returns an error. Also, in some places I `throw` errors. How safe is it?

![screenshot 2014-07-15 21 33 14](https://cloud.githubusercontent.com/assets/1079123/3594643/27d3d788-0ca3-11e4-8a80-223c2955c38b.png)

No, not all of them. I just thought that it would be nice to have more control. I'll remove the optional ones. However, it's still not clear to me how Bugsnag would detect if a value in the field is incorrect. For example, if entered an incorrect product name, a Bugzilla API would return an error. How would Bugsnag handle this?