Updating to work with celery 3.x

celery 2.3 had a state object declared in the state module:
https://github.com/celery/celery/blob/v2.3.0/celery/events/state.py#L314

celerymon depended on this, but it was removed in a later version of celery. This pull request adds celerymon's own state object.

It also updates the api's json decorator to unroll generators