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 was talking about `delete()` ... Point being, `delete()` may fail after `sRemove()` has succeeded. This should do it:

    if ($this->_redis->delete($key) === 1 && array_search($key, $this->_serialized, TRUE))
    {
        $this->_redis->sRemove('_ci_redis_serialized');
        return TRUE;
    }

    return FALSE;