Tests and a couple of new features

I like the way this library is designed, and I'm going to be using this library for project at work, so I added some tests, and a couple of features.  

I'll probably be contributing more in the near future as I need more functionality.

Please let me know if you're not interested in maintaining an active fork any more.

Details:

* Test, mocha + chai.  Unit tests for most functionality, coverage looks good but there are still some cases that    should be added, I think.
* Queue methods: getAttributes, setAttribute.
* Queue property to disable JSON parsing of message bodies.  This is a hack, and I'm going to open an issue to do it better somehow, but for now it's nice to have.