Update updatecheck.py

Check for the addition of an icon_hash key in a package plist. If the key is present and matches a hash of an icon that has already been downloaded no attempt will be made to download the icon again. If the icon_hash is not present or the hash does not match the behavior will revert back to normal.
If you revert this bit, I'll merge the other changes; we can then discuss/work on the documentation and/or munkiimport changes seperately.