Queue allow to control the execution of a list of promises.
Especially, it can cancel the list of promises.

The queue is fulfilled only when all pushed promises are fulfilled.
It gets the latest push promise fulfillmentValue as result.

    var queue;
    queue = RSVP.Queue();
    queue.push(function () {
      return 1;
    };
    queue.push(function (value) {
      return value + 1;
    };
    queue.then(function (value) {
      assert(value === 2);
    };