SPARK-2298: Show stage attempt in UI

Added attempt ID column into stage page of webUI.
Added attemptId handling code into StageInfo, JsonProtocol.
Modified DAGScheduler to identify stages whose stageId is same but attemptId is different.
Modified testcode for stage attempt ID.
Ah, no it's not. How about this?
```
  val attemptId = (json \ "Attempt ID").extractOpt[Int].getOrElse(0)
```