deferred API: local executor

[jknack]

In addition to the #488 which add a global executor to Deferred API a local (per call) executor is will be available too:

{

  // global/default executor
  executor(Executors.new...());

  // cached executor
  executor("cached", Executors.new...());

   get("/global", promise(deferred -> {
     deferred.resolve("OK");
   }));

   get("/local", promise("cached", deferred -> {
     deferred.resolve("OK");
   }));
}

The Deferred class now takes a String which works as an executor reference:

new Deferred("cached", ...);

The executor method call is a shortcut for:

binder.bind(Key.get(Executor.class, "cached")).toInstance(executor)