Use local node packages (defeatureify & yuidocjs).

* Moves `defeatureify` into `/node_modules`.
* Moves `yuidocjs` into `/node_modules`.
* Add helpful error message when the required version of `defeatureify`
  is not present.
* Resolves a long standing issue with using `which` (doesn't work on
  Windows).

Resolves #97.
Resolves #54.