Add helper functions for client API

The Razor::View module provides Sinatra helper functions to render database
objects as hashes, which can then be returned to the client as JSON.

By making this functionality available as helper functions instead of wrapper
classes, the functions have access to other helper functions like compose_url,
and to each other. This permits, for instance, policy_hash to provide links to
Razor::Data::Tag objects by calling tag_url.

As suggested by @daniel-pittman, this link-based API format allows clients to
retrieve linked objects without building a URL. The linked object format is
intended to conform to RFC5988.
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.