Parsing helpers for plugins

Plug-ins got a bit more complex when we added Py3 support. Cross platform plug-ins needed to do:
	from zmq.utils.strtypes import u
	import zmq.utils.jsonapi as json

Then in the handle_recv, plug-ins needed to do:
	topic, msg = data
	topic_parts = u(topic).split('.')

If you want the dict from the JSON stored in msg, you needed to use the loads from zmq.utils.jsonapi.

This seems like to much to need to know. So instead, I introduced a couple helper methods to the CircusPlugin class and updated the docs to show their use.