Add tlsdate

This adds tlsdate to homebrew. In theory release 0.0.6 will support
OS X out of the box and so I have included the latest tagged release
as well as a --HEAD target. One must use --HEAD today on OS 10.8.2
or better.

When this patch is merged, I will update the tlsdate documentation
in master of the tlsdate git repo to instruct people to install
tlsdate with homebrew rather than doing it by hand.
use the release tar.gz for tlsdate stable