Add GNOME target

Unity got you down? Has XFCE thrown a brick through your window? Not kool enough for KDE?
Then I've got the pull request for you!

This adds a GNOME shell target to crouton (`-t gnome`), and the necessary host-bin/chroot-bin scripts to launch it. If you've got good ol' fashioned xorg, you'll get GNOME shell in its full glory; xephyr should fall back to GNOME panel automatically.

I've tested it on arm/i686/x86_64, but moar testing is moar better! Any feedback would be great.

KNOWN ISSUES: Installing the gnome target in a Debian chroot will currently abort the the install; you should be able to go ahead and `enter-chroot` and continue the setup script just fine. I'm working on a patch for this bug but that'll be in a separate pull request (follow #279).

This pull request also finishes the second half of issue #6.