Prune dirlist before descending into sub-directories

Hey man, loving the asyncfinder :D

But I discovered it is visiting every file and folder under supposedly "ignored" folders (then filtering them out afterwards).  I found how to fix that!

Please see my latest commit: b8dcdc6

"prune dirlist before descending into sub-directories, rather than visiting lots of unwanted folders and filtering the results out later"

You probably are less interested in my earlier commits, except for verification.  They were what I used for debugging when I found the searches were going slower than they should.