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.

- [x] Pass all unit tests for basic functionality.
- [x] Integrate Redis Extension into Travis-CI testing.
- [x] Add support for multiple servers, using the RedisArray class.
- [x] Revisit serialization option to see if it can be improved.
- [x] Improve test coverage with driver specific tests.