[SPARK-2297][UI] Make task attempt and speculation more explicit in UI.

New UI:

![screen shot 2014-06-26 at 1 43 52 pm](https://cloud.githubusercontent.com/assets/323388/3404643/82b9ddc6-fd73-11e3-96f9-f7592a7aee79.png)

Author: Reynold Xin <rxin@apache.org>

Closes #1236 from rxin/ui-task-attempt and squashes the following commits:

3b645dd [Reynold Xin] Expose attemptId in Stage.
c0474b1 [Reynold Xin] Beefed up unit test.
c404bdd [Reynold Xin] Fix ReplayListenerSuite.
f56be4b [Reynold Xin] Fixed JsonProtocolSuite.
e29e0f7 [Reynold Xin] Minor update.
5e4354a [Reynold Xin] [SPARK-2297][UI] Make task attempt and speculation more explicit in UI.