Add support for OS X's md5 utility
Add support for OS X's md5 utility

This change checks for LSB's md5sum utility and OS X's md5 utility (in
that order) and uses whichever it finds.

bitmonk/fabtools,pombredanne/fabtools,fabtools/fabtools,ronnix/fabtools,ahnjungho/fabtools,AMOSoft/fabtools,sociateru/fabtools,wagigi/fabtools-python,davidcaste/fabtools,pahaz/fabtools,n0n0x/fabtools-python,hagai26/fabtools,prologic/fabtools,badele/fabtools