Add policy slice to CLI client

The policy slice closely resembles the tag slice, and is currently capable of
listing policies.

Because policies are more complex than tags -- they include nested objects
like image and tag references -- Razor::CLI::Slice#print_objects was modified
to detect and print these special objects. Notably, it recognizes the
"object with a 'url' key" pattern, and prints it as such. Object URLs are
printed as paths relative to the server root (no scheme/port/hostname) for
brevity.