Bugzilla plugin is now converted to backend sync service

as the summary, Bugzilla is now a backend sync service. It can be used simply as it was a plugin before. When enable this service, any added task will be handled and attempt to sync bug information from specific related Bugzilla service.

This version works well, but not perfect. I'm being continue to add more features, such as the configuration ability.
Have you heard of [YAGNI](http://en.wikipedia.org/wiki/You_aren't_gonna_need_it)? Besides, `BackendSignals().backend_failed()` is already the mechanism how to handle errors and communication between backend and UI.

This code belongs to only single object and it should be moved there