Promise Free Async/Await

I've never used co but looking it up, it seems to be more about promises and thunks.

This one is just Plain Old Callbacks. The goal was to be promise free, continuation passing style.

I just did some benchmarks and it's also more than 2x as fast as native async/await (I updated the readme with results).

I added a 'Why' section to the [readme](https://github.com/bessiambre/casync). Maybe this helps understand the reasoning better?