Support partial function to map message to hash key, see #944

* Partial function, ConsistentHashRoute, for Scala API
* withConsistentHashMapping ConsistentHashMapping for
  Java API
* Updated documentation

akka-docs/scala/code/docs/routing/ConsistentHashingRouterDocSpec.scala

@@ -19,9 +19,12 @@ object ConsistentHashingRouterDocSpec {
     def receive = {
       case Entry(key, value) ⇒ cache += (key -> value)
       case Get(key)          ⇒ sender ! cache.get(key)
+      case Evict(key)        => cache -= key
     }
   }
 
+  case class Evict(key: String)
+
   case class Get(key: String) extends ConsistentHashable {
     override def consistentHashKey: Any = key
   }