Add ability to ignore files by shell pattern
Be forgiving of empty lines in checksum file
Fix for Python 3

In py3k, dict.keys() returns an iterator, so it's invalid to mutate the dict
while iterating (in python2, it was a true list). Pass it through the list
constructor to force it into a real list.
Make ignorelist actually optional (oops)
Add --clean-ignored to clean ignored cruft from db