Redesign high-level API

[panicbit]

The original API has been written a few years ago when I was still getting started with Rust, so it probably doesn't reflect past or current best practices.

Ontop of that, the API currently only covers basic usage.

[lutter]

I agree with that sentiment.

What I'd like to get to is a lower level API, that's a pretty direct translation of the C API (along the lines of PR #10, though I am not sure how rustaceous that is) and a more comfortable API for traversing the tree along the lines of what I did in mruby That should internally still be based on string paths, but it will require something like an Rc<Augeas> to get memory management right.