Added headers argument support to get_object()

With this change conditional and range GETs are possible, as
documented in official API guide:
http://docs.openstack.org/api/openstack-object-storage/1.0/content/retrieve-object.html

Change-Id: Ib2ed1c21e8d3f1ed79c0b7e542ee022ee535835c
Merge "Added headers argument support to get_object()"