Read from replica

Hi, Alexander! 

I have made distributed installation of  `cacheops` on my production servers. Configuration is master-slave on top of general RedisDB replication. It looks good for me. At present moment it have been working during several days.

I could divide load between two instances. You can see, red and blue graphs on image below

![screenshot-z rutube ru 2014-11-10 20-32-36](https://cloud.githubusercontent.com/assets/1039574/4980363/ab814eae-68ff-11e4-913f-43d8c18130c6.png)

Early replica was "cold" and used only as fallback (blue line). Now all reads serves by both instances. CPU usage decreased from about 95% at peak to 60% on each servers.

Hope my changes help to solve part of the problem #35. Please check them and accept if you will find them useful.