[WIP] Redis Driver

This PR is for a Redis Driver using the PhpRedis extension. It was requested as part of issue #78.

 It is currently a work in progress and should not be merged yet.

- [ ] Pass all unit tests for basic functionality.
- [ ] Integrate Redis Extension into Travis-CI testing.
- [ ] Add support for multiple servers, using the RedisArray class.
- [ ] Revisit serialization option to see if it can be improved.