Adding formulas for ndenv and node-build. Inspired by rbenv and ruby-build formulas.

These formulas enable installation of ndenv and node-build, an alternative to nvm for managing multiple installations of nodejs.
That came from the ruby-build and rbenv formulas, I was basically satisfied to se that the script ran once installed. Now that I think about it, that would make it a pretty useless test since it doesn't actually verify anything. I think I'll remove it until I have an idea on how to actually properly test it.