representation for Ref objects

The ref objects were missing a python __repr__, ended up with a not so
useful trace in the debug.log. This creates a meaningful representation
for human consumption:

	<Ref 0x7f51f848ffd0 creates refs/tags/v2.0.0 on 4eb656fe>
	<Ref 0x7f51f8491990 refs/tags/v2.0.0 updated 4eb656f..2260abb>
	<Ref 0x7f51f848f610 deletes refs/tags/v2.0.0 from 2260abb>

(sha1 above have been made shorter than the actual output)

Change-Id: If3b60494f1984a04e9ffada38f7e3b99f2a8ba0e
Reviewed-on: https://review.openstack.org/19925
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins