Implement getBindings

I want to get bindings for a scope and the only way to do it is: `scope.declares('') && scope.bindings` to force it to scan. So I added a public `getBindings` method to do that.

Also, I was almost halfway implementing my own scope thing when I found out that `ast-types` had support for this (which is awesome!) so I added a mention in the readme, but really, this project (and recast) needs some documentation love. I still don't fully understand NodePath (especially with all the `defineProperty`). It could be just quick class, method, params documentation. Thoughts?