Add elements to json for bindings to default exchange

When generating graphs I was seeing that queues without bindings were being graphed oddly. In my usage of rabbitmq the way they would get messages would be by publishing to the "" exchange with their queue name as the binding key.  

This patch adds appropriate data to the json structure to produce that in the graph. I'm not proficient at python, so I don't know how to modify the definitions data in a function other than to make it a global.  If you have ideas about the code, let me know.  Thanks for the graphs BTW!