only allow snapshots of overlayfs backed containers

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Make restore a snapshot as well.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
make install to $DESTDIR/bin

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
update README

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
update README again

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>