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 and the terminal-tables gem (for listing replies).
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. These classes use a simple syntax to define
both a CLI command and its associated endpoint. The result is a one-to-one
correlation between a recognized endpoint and its associated class method.

Command endpoints are recognized by the path of the "rel" parameter in the
objects returned by /api. This allows for unique identification of endpoints
while following the (non-)standard Razor implementation of "rel" (which lists
the spec location as relative to the current server, instead of as a single
canonical URL).