update actor doc and add more actor based tests

tests/actor_send_reply.ce

@@ -0,0 +1,9 @@
+// Test: send with reply callback
+$start(function(event) {
+  if (event.type == 'greet') {
+    send(event.actor, {ping: true}, function(reply) {
+      if (!reply.pong) disrupt
+      $stop()
+    })
+  }
+}, 'tests/actor_helper_echo')