Bug 1030972 - Import tasks from Bugzilla

@bobsilverberg @Osmose r?
It could be an instance method, though, or so it seems. Do you foresee calling this directly on the class and not via an instance? If it were an instance method then it could use `self.query` instead of passing the query around as an argument. This seems better and more logical to me. Unless I am misunderstanding it seems like you would always be calling the method in the context of a batch anyway.