Create CLI client

This is a command-line client for Razor that uses the public API to retrieve
(and eventually control) a Razor server. It is currently capable of listing
tags found on the server, but is easy to extend.

Because the client interacts only through the public API, it requires only a
basic Ruby environment, the terminal-tables gem (for printing replies), and
the rest-client gem. This allows the client to be used remotely as well as
locally, and avoids the performance penalty associated with loading the entire
Razor codebase.

Commands are grouped within slices (for lack of a more original name), which
inherit from Razor::CLI::Slice. Slices are invoked through their #run method,
and should list explicitly the commands they respond to.

Command endpoints are recognized by the path of the "rel" parameter in the
objects returned by /api.
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.