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.
I'm not getting you here. On one side, you want to have a generic interface that allows you customisation, e.g. include also field *XYZ* in the comments for my field. On the other hand, you removed the explicit API and replaced it by meta-code. Decide what you want to achieve:

 - if you want extensibility, clearly define interface that could be implemented differently for different bugzillas
 - if you want brevity of code, just use `self.bug` object directly