Split out from #534.

Details:

• Return a promise from a test
• That promise should be automatically asserted to fulfill
• Automatic async start/stop

Usage:

QUnit.test("promises promises...", function(assert) {
  assert.expect(1);
  return $.ajax("/api/blah");
});

Plan is to use a similar patch to what @domenic mentioned doing for Mocha: #534 (comment)

CC: @stefanpenner @domenic