Use cross-spawn to spawn processes

[bdukes]

This provides better cross-platform support

For example, on Windows, I can now add graphicsmagick-binary-win64 as a dependency and then call gm(path).options({ appPath: 'node_modules/.bin/', }) and use gm without having to make sure that GraphicsMagick is installed globally on whatever machine I'm using.

Thanks for your consideration!

[aheckmann]

thanks!