Fix npm executing on windows, spawn of npm fails with ENOENT

zelda fails to invoke npm install on windows.

The underlying [issue](https://github.com/joyent/node/issues/2318) is node's executable resolving algorithm, which I in doubt will be fixed soon.

More about it [here](http://stackoverflow.com/questions/17516772/using-nodejss-spawn-causes-unknown-option-and-error-spawn-enoent-err#answer-17537559). 

Other possible fix is to [replace spawn with exec](https://github.com/yurynix/zelda/tree/feature/replace-spawn-with-exec), but that'll ruin npm's color output.