Allow compass compile to traverse symlinks. Fixes #1370

`**` in `Dir.glob` does not traverse symlinks http://stackoverflow.com/questions/357754/can-i-traverse-symlinked-directories-in-ruby-with-a-glob. 

Sprokets ran into the same issue https://github.com/sstephenson/sprockets/pull/189. I've ported their patch to compass.

I'm not a ruby developer so I imagine this could be made more terse.