3.6 manifest split out into separate git module

[Colin Walters <walters verbum org>]

I've just split out the gnomeos/ directory (i.e. the manifest and pile
of patches) into:

http://git.gnome.org/browse/gnome-ostree

Which is targeted now at 3.6.  I don't think anyone's going to
be interested in 3.4 for long, but if you do care, the old
code remains on the "v2012.6" git tag in ostree.

I've kicked off a resolve+build of 3.6 on ostree.gnome.org now.

This helps ensure a clearer separation of "ostree the tool" from
"gnome-ostree the operating system", among other things.