Make a Signal forget its internal memory when stopped.

This branch changes Signal.onStop to make it forget its
internal memory when all observers have been removed. Causing it
to re-compute its "initialValue" again if the Signal is re-started.

This way, dependent signals (eg, MapSignal) can ask their parent
signal for the latest value upon restarting, and elements that read
these dependent signals can now observe the correct state when re-mounted.

Fixes raquo#43

src/main/scala/com/raquo/airstream/signal/Signal.scala

@@ -137,6 +137,11 @@ trait Signal[+A] extends Observable[A] {
     super.onStart()
   }
 
+  override protected[this] def onStop(): Unit = {
+    super.onStop()
+    maybeLastSeenCurrentValue = js.undefined
+  }
+
   // @TODO[API] Use pattern match instead when isInstanceOf performance is fixed: https://github.com/scala-js/scala-js/issues/2066
   override protected def onAddedExternalObserver(observer: Observer[A]): Unit = {
     super.onAddedExternalObserver(observer)