Add a redis driver

Add the foundations of a redis driver that can be used
along side the rest of the driver types.

Redis provides a few nice benefits that act as a poormans
zookeeper.

- Durability (when setup with AOF mode).
- Consistent, note that this is still restricted to only
  one redis server, without the recently released redis (alpha)
  clustering > 1 server will not be consistent when partitions
  or failures occur (even redis clustering docs state it is
  not a fully AP or CP solution, which means even with it there
  will still be *potential* inconsistencies).
- Master/slave failover (when setup with redis sentinel), giving
  some notion of HA (values *can* be lost when a failover transition
  occurs).

Further resources/links:

- http://redis.io/
- http://redis.io/topics/sentinel
- http://redis.io/topics/cluster-spec

Change-Id: I2d90be1afae98f40ff5c4230563e686b23d42c8f
Merge "Add a redis driver"