#3843 Add ClusterSingletonProxy

This is my draft of the new Proxy class. It uses idea from the docs as well as things already used in the SingletonManager and finally ideas taken from my discussion with Patrik Nordwall on Assembla. I think this class is already usable although it might need a few more tests and the more experienced eyes of the Akka devs (I'm quite new to Akka). 
I'll be happy to make any suggested changes, improvements, etc.