Documentation Update and Cleanup

With Resque 2.0, I found much of the older documentation and examples to be woefully out of date, and the ordering of the main `README.md` to be counter-intuitive and to have made a lot of assumptions about user knowledge. My students hit big issues using Resque 2.0, and much of the 1.x documentation found online didn't help. 

So I updated several of the example files, the README.md and CONTRIBUTING.md. Many of the examples in the readme didn't work, and contributing needed a few minor changes. 

No changes to the core code were made and all tests that were passing prior are still passing.