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 this test be modified to do something more substantial than e.g. `--version` or `--help`? See `cmake.rb` for an example of an application formula with a good test and `tinyxml2.rb` for an example of a library formula with a good test.

Also, please use `system` instead of `exec` and put the separate arguments into separate strings.

Thanks!