Add Consistent Hashing router support

[rogeralsing]

http://doc.akka.io/api/akka/snapshot/index.html#akka.routing.ConsistentHashingRouter

There is 3 ways to define what data to use for the consistent hash key.

1. You can define hashMapping / withHashMapper of the router to map incoming messages to their consistent hash key. This makes the decision transparent for the sender.
2. The messages may implement akka.routing.ConsistentHashingRouter.ConsistentHashable. The key is part of the message and it's convenient to define it together with the message definition.
3. The messages can be be wrapped in a akka.routing.ConsistentHashingRouter.ConsistentHashableEnvelope to define what data to use for the consistent hash key. The sender knows the key to use.

[rogeralsing]

For implementation, see:

https://github.com/akka/akka/tree/master/akka-actor/src/main/scala/akka/routing
https://github.com/akka/akka/blob/master/akka-actor/src/main/scala/akka/routing/ConsistentHashing.scala

I guess the easiest way is to copy RoundRobinGroup and reimplement the *Logic class to deal with the 3 above listed ways of hashing.
But try to keep close to the Akka impl

[rogeralsing]

Some initial support added:
We now support consistent hashing through:

• Messages implementing ConsistentHashable
• Wrapping messages in a ConsistentHashableEnvelope message

via ConsistentHashGroup router.

[rogeralsing]

Added pool support also.

Can't mark this one as complete though, there are alot more magic to consistent hashing, Akka has virtual nodes and other features we need to deal with.