Adding Java API for reduce, fold, apply and firstCompletedOf, adding << and apply() to CompletableFuture + a lot of docs

akka-actor/src/main/scala/akka/japi/JavaAPI.scala

@@ -7,6 +7,13 @@ trait Function[T,R] {
   def apply(param: T): R
 }
 
+/**
+ * A Function interface. Used to create 2-arg first-class-functions is Java (sort of).
+ */
+trait Function2[T1, T2, R] {
+  def apply(arg1: T1, arg2: T2): R
+}
+
 /** A Procedure is like a Function, but it doesn't produce a return value
  */
 trait Procedure[T] {