Cleanup green implementation of ZMQIOLoop.instance() and friends.

The green ZMQIOLoop class can safely inherit instance(), install() and initialized() methods from its superclass.  AFAICT making the superclass instance() method a @classmethod rather than a @staticmethod means it will Just Work without any further monkeying in the green subclass.

This seems to fix some weirdness I was seeing, where tornado.ioloop.IOLoop.instance() would return the wrong class.