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-remote/src/main/scala/akka/remote/RemoteActorRefProvider.scala

@@ -138,7 +138,7 @@ class RemoteActorRefProvider(
 
   def actorFor(ref: InternalActorRef, path: Iterable[String]): InternalActorRef = local.actorFor(ref, path)
 
-  def ask(result: Promise[Any], within: Timeout): Option[ActorRef] = local.ask(result, within)
+  def ask(within: Timeout): Option[PromiseActorRef] = local.ask(within)
 
   /**
    * Using (checking out) actor on a specific node.
@@ -160,12 +160,12 @@ trait RemoteRef extends ActorRefScope {
  * This reference is network-aware (remembers its origin) and immutable.
  */
 private[akka] class RemoteActorRef private[akka] (
-  override val provider: RemoteActorRefProvider,
+  val provider: RemoteActorRefProvider,
   remote: RemoteSupport[ParsedTransportAddress],
   val path: ActorPath,
   val getParent: InternalActorRef,
   loader: Option[ClassLoader])
-  extends InternalActorRef with RemoteRef {
+  extends InternalActorRef with RemoteRef with ActorRefWithProvider {
 
   def getChild(name: Iterator[String]): InternalActorRef = {
     val s = name.toStream