Source images directory management

Hello there,

The problem with how is currently handled the spriting process is that every image is in the app/assets/images directory. This leads to unneededly huge assets:precompile which compiles *every single image* when it actually usually only needs to compile the sprite.
There was no easy configuration in sprockets side to choose which folders should be pre-compiled and which shouldn't which leaded me to this pull request.

Just for a quick benchmark, our precompilation process fell down from 2 minutes to 10 seconds !

I've added this as a config option so that it would be a totally transparent change.