Use a binary shim

I quite like the idea of using a binary shim, instead of manipulating the PATH and copying excutables back and forth.

Also it enables a feature that I love from [nodenv](https://github.com/OiNutter/nodenv): Put a `.node-version` file in any directory and the node version specified in there will be used in that directory and all its subdirectories automatically.

This pull request adds a binary shim written in go that proxies *all* calls to node, by dispatching the versions specified in env vars, local `.node-version` files, and the global `.node-version` file.

Additionally it allows you to add custom cli params statically for specific version, that will be applied automatically. (E.g. you can tell nodist to always use `--harmony-generators` with node 0.11)

Feedback is greatly appreciated!