Support authentication when pulling images

Pulling from a private registry that requires authentication
was not supported.  Use the X-Registry-Auth header to pass
authentication credentials.
Use X-Registry-Auth header for PushImage auth

To be consistent w/ PullImage and this seems to be the preferred
method for sending registry auth credentials since the 1.5 version
of the docker remote API docs.
authors: add Jason Wilder
client: do not check for nil value in map before ranging it