Project housekeeping

Update Dockerfile to use @tiborvass fork of libgit2.

Signed-off-by: Arnaud Porterie <arnaud.porterie@docker.com>
Change Scope semantic by allowing chaining

Chaining scopes is now possible in such way that
`root.Scope("a").Scope("b").Set("foo=bar") now creates key "a/b/foo".

Signed-off-by: Arnaud Porterie <arnaud.porterie@docker.com>