[Gtkradiant] Now on github!

[Timothee Besset]

The code is now on github: https://github.com/TTimo/GtkRadiant

Hopefully that will make it easier to review and accept patches. I
will retire the code SVN soon.
The assets used in the game packs will probably remain on the SVN that
is hosted on icculus.org for the time being.

I want to details a few things moving forward, but it's getting late,
so some other time.

TTimo