Initial work on RC checker

Initial work on combining MD5 and signature verification into a single script.

Still needs quite a bit of work. I'm not happy with it needing an external script for MD5 verification but for some reason the default MD5 on 10.9.2 (I think that's what I'm on) can't verify a signature?? (Seriously, am I dumb or something??). GNU md5sum would work as well but that's also not installed by default on 10.9.2. For now, md5deep is easy enough to install (especially with Homebrew) so it should do as a first pass.

The signature verification is a bit brittle with its assumption that every release will be available as a *.zip. Should really check for one of *.zip, *.tgz, *.bzip, etc. and try to import the keys from one of them. Should be a good enough for now though.