Fall back to md5sum if md5 is not available

Thanks for your script! According to issue #8 it was designed to work on Mac and does not (yet) support Debian/Ubuntu-based systems.

These changes makes this script run out-of-the-box on any recent version of Debian/Ubuntu, whilst keeping compatibility with the current approach. I've confirmed this to work on Ubuntu 12.04 and 14.04. I don't have a Mac to test against, so any feedback on compatibility is appreciated.

Fall back to md5sum if md5 is not available
Make gravatar check work on Debian/Ubuntu-based systems

Fixes #8