Add an .md5 reference for bins

This addresses #23 partially by providing a trusted md5 check file.

#23 will be resolved completely when multidoge.org, the main distribution site for the binaries generated here, has SSL.

While users could be fooled into downloading a malicious multibit binary, they can at least refer to this md5 check file, since it's stored in a secure location -- GitHub repos do require some kind of authentication, and are themselves always-SSL.