Cache_redis: Adding serialization support.

This is an alternative implementation of value serialization for Redis cache driver. It is the third idea that appeared at the discussion about https://github.com/EllisLab/CodeIgniter/pull/3181

Now the driver has an internal cache for storing keys of serialized values, in order these serialized values to be distinguished.

I tested it with the same test of mine, it looks OK.
I removed both of the array_search checks and my test failed. There is nothing to be removed here.

![failed_redis_test](https://cloud.githubusercontent.com/assets/1438104/3949297/48d13838-26b4-11e4-9a1b-f47a90395afa.png)

When I brought back the array_search() checks, the test got OK.