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 since md5deep is able to support multiple aggregate md5 file formats. This allows us to potentially avoid a situation where the person rolling the release used md5deep and the person verifying the release ends up trying to test with md5sum. md5sum is not a big fan of the md5deep output format, but the other isn't true. Obviously this is still possible if both both applications aren't installed on the tester's machine, but this will avoid the problem if at all possible. We should also update the RC rolling script to prefer md5sum over md5deep if possible as well.