Failover detection

Hi there,

I recently had a requirement to extend the redis cache module to support automatic failover to another server, I've implemented this via the phpredis module, and thusly have a check wrapped around the code to make sure this exists in the php build. Main reason for this direction is I didn't have the time to learn how it works, and any overheads that would be tied to instantiating the credis_client class twice for the check. This code does add an additional overhead, so it should really only be turned on if it is needed.

Thanks!