Allow compass compile to traverse symlinks. Fixes #1379

Sometimes asset pipeline setups require directories to be symlinked from a working directory to a public directory, for example when using mixins defined in bower'd components.

Compass currently doesn't support this because `Dir.glob('**')` does not traverse symlinks to prevent infinite loops. 

Sprokets had a similar issue and solved it by allowing symlink traversal for up to one level https://github.com/sstephenson/sprockets/pull/189. I've ported their patch to compass.

I'm not a ruby developer by trade so I image this could be made more terse.