fixes #10 by trying md5sum, then md5, then falling back to SWI-Prolog's in-memory MD5 implementation
fixes the -l option (scan directory outputting metadata) which I don't quite understand the use case for but fixed anyway since I think I broke it at some point ;)
updated the examples directory and the README to reflect some relatively minor (IMO) changes I had to make to bring syntax in line with GNU make
-loption (scan directory outputting metadata) which I don't quite understand the use case for but fixed anyway since I think I broke it at some point ;)