Support for user namespaces

Support for UID namespace is available in the 3.13 kernel.  This change
exposes the support via a new command line option (--uidmap) to run an
application with a virtual UID namespace where the application could
be a root in the container but unprivileged on the host.

Docker-DCO-1.1-Signed-off-by: Dinesh Subhraveti <dineshs@altiscale.com> (github: dineshs-altiscale)