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).
Make it possible to disable the comments section entirely without having to muck with theme files.
Adding a default for comment disabling.
Merge pull request #1048 from MrJoy/simplify_clean

Simplifying clean task.
Merge branch '2.1' of git://github.com/imathis/octopress into testing-2.1
Merge pull request #1066 from MrJoy/enable_disabling_comments

Make it possible to disable the comments section entirely without having to muck with theme files.
Merge branch '2.1' of git://github.com/imathis/octopress into testing-2.1
Update Octopress.