polish “ask” pattern, see #1581

- move package objects into their respective package.scala file in the
  right directories
- make implicit conversion as well as explicit facility available under
  the same name akka.patterns.ask for easy import
- revert the logic to produce the Promise for the PromiseActorRef within
  the ActorRefProvider; supporting wrapping of external Promises does
  not seem to justify doing needless extra allocations in case of
  failure
- add scaladocs
- factor out “def provider” into trait ActorRefWithProvider, as it
  didn’t feel right attaching this information “by exception” to
  MinimalActorRef

akka-docs/scala/code/akka/docs/actor/ActorDocSpec.scala

@@ -8,7 +8,6 @@ import akka.actor.Actor
 import akka.actor.Props
 import akka.event.Logging
 import akka.dispatch.Future
-import akka.Patterns
 
 //#imports1
 
@@ -221,6 +220,8 @@ class ActorDocSpec extends AkkaSpec(Map("akka.loglevel" -> "INFO")) {
 
   "using ask" in {
     //#using-ask
+    import akka.patterns.ask
+
     class MyActor extends Actor {
       def receive = {
         case x: String ⇒ sender ! x.toUpperCase
@@ -230,10 +231,10 @@ class ActorDocSpec extends AkkaSpec(Map("akka.loglevel" -> "INFO")) {
 
     val myActor = system.actorOf(Props(new MyActor), name = "myactor")
     implicit val timeout = system.settings.ActorTimeout
-    val future = Patterns.ask(myActor, "hello")
+    val future = ask(myActor, "hello")
     for (x ← future) println(x) //Prints "hello"
 
-    val result: Future[Int] = for (x ← Patterns.ask(myActor, 3).mapTo[Int]) yield { 2 * x }
+    val result: Future[Int] = for (x ← ask(myActor, 3).mapTo[Int]) yield { 2 * x }
     //#using-ask
 
     system.stop(myActor)