github port group

[Sean Farley]

Following the review here:

https://trac.macports.org/ticket/33889

How should the port group use a git hash for determining which version
to download? Some features that would be nice:

1) ability to send a hash for the version to download (without having
git as a build dependence); this would make it possible to cache the
download as opposed to cloning every time

2) using the rss for livecheck (allows only checking the newest commit
hash via the regexm command)

3) uniform versioning

For (3), I prefer ${last_known_version}-${date} but don't mind
changing this to something else as long as it's consistent.

Open question: what to do about projects that never version? Assign a
dummy version? Stick with just date?

Again, I don't mind so much as long as there is something consistent.