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 the Bugzilla backend, a cool feature is to allow users to manage their frequently-used Bugzilla services, whereas Bugzilla backend just provides several basic and popular ones. This file would be removed when achieve that goal.

The big reason why I wrote these code in this file for now, is for extention to handle potential different, there is, bug serialization result from different Bugzilla service. I treat those concrete class inherited from Bug as a placeholder. This is much clear for other developers, including me of course :), where is proper to add code aiming to specific Bugzilla service, rather than if-elif-else in Bug class. I really don't like that way.

Anyway, I modified ``create`` method as a module level method by dropping BugFactory class.