Simplifying clean task.

* Make it less needlessly noisy (no need to enumerate all files when doing rm_rf).
* Explicitly nuking screen.css is superflous as `compass clean` will do this for us.
* Make task description more accurate about what's happening.
* Make task feedback less redundant (details of what was removed are right above it).
Merge pull request #1048 from MrJoy/simplify_clean

Simplifying clean task.