Update verify-md5 with better cross platform support

- verify_md5_checksums now checks the user's system for a usable md5 application instead of defaulting to md5sum. This helps to address the issue with OS X 10.9.x not having md5sum installed by default.
- When given a choice between md5sum and md5deep we prefer md5deep. md5sum seems to have issues verifying md5deep outputs whereas the other way around is not a problem.