Reduced Code, Custom Message Properties

This release sees major cleanup and consolidation of the code behind all of the producer/consumer pairs in Rabbus. The vast majority of the code has been reduced to the base producer / consumer objects, with each specific pair being configuration with appropriate method calls on top of them.

This will open opportunity for simplified code overall, and moving forward with a better API - more node-like w/ callbacks that get error as first param, etc.

Additionally, message producers can now publish custom message properties. See the readme for information on this.