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)

Not sure what you mean. Take a look at line #40 

https://github.com/bugsnag/bugsnag-notification-plugins/pull/79/files#diff-b100add6df243059eeff223043347606R40