Modify grabNewImages to follow symbolic links and ignore hidden files
Simplify the hidden file skipping

The for x in (...) code looked a bit tricksy, so just skip the .file
inside the regular loop.
Add a test for the hidden file ignoring
Some MP4 files seem to be allowed too
Sets created during pass one shouldn't be created again at the end

Add a test for this too. Fixes issue 35.