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.
Can't install things into the prefixes of other formulae or they will be broken on upgrades. You should create a shared plugin directory and symlink to it instead. Also, you can't rely on `HOMEBREW_PREFIX` being `/usr/local` here.