Adding Sentinel Commands

[therealbill]

This PR adds a new set of methods/commands used for talking to a Redis Sentinel server. It introduces no dependencies, has tests for the non-trivial methods, embedded doc comments, and an additional markdown file detailing usage of the new methods.

This new set of supported commands are useful in interrogating a Redis sentinel as well as implementing Sentinel support (either in client code or in the library itself).

Note, this implements the sentinel aspect of issue #7.