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.
To first suggestion, this way give a good abstraction and code structure. backend_bugzilla.py should contain configuration and control code just for the backend itself. Then, the real work should go to the backend/bugzilla package. No matter how complicated Bugzilla backend will be or just keeping simple. This code structure makes thing much clear.

To second suggestion, good idea. will consider this later. For now, I would like to keep Bugzilla backend as simple as possbile, either from the design or implementation. Let it work, then enhance. What do you say :)