Adds Support for Redis 2.6.x and Redis-Sentinel

- Removes VM configuration options which are no longer present in Redis 2.6.x

- Adds support for redis-sentinel monitoring. Including the ability to fully configure sentinel with attributes.

- Adds example roles configuration

- Updates README

- No longer runs make install as the redis user. (useful if you, like me, like installing into /usr/bin)

Caveat: Sentinel is untested with the package installation method. It *should* work (on any package that includes a new enough redis version) but I have not personally tested it, so I have opted to raise an error if you're using sentinel but with the package option. It would be great for someone in this scenario to verify package installation works, and pull request a change that removes the raised error.