Add 'nodes' function to return the syntax tree

I have a use case (https://github.com/eapache/starscope/issues/26) where I would like access to the coffeescript syntax tree from Ruby. This PR adds a `nodes` method almost exactly like the `compile` method, just calling the underlying `nodes` function instead to return the syntax tree.

I hacked this together quickly via copy-paste, not knowing much about how the bridge actually works. It probably needs refactoring (`compile` and `nodes` share a lot of common code) and of course it needs some tests. I just wanted to get an acknowledgement that my approach generally makes sense and that this is acceptable before I do that work.