Huge improvements, read extended please.

Split excluded into excluded_folders and excluded_files. Added
imagesets with a default value of ['.imageset']. Add them here, if you
use several Image Sets.
Properly searches and deletes imagesets.
Properly ignores appiconset and launchimages.
Properly searches for name occurrences: removed the .downcase to be
more exact. No idea why I used it, it was just dumb; other minor
improvements too.
It now removes references to the deleted (stand-alone) images from your
pbxproj file; imagesets are automatically handled by Xcode.