[SPARK-3957]: show broadcast variable resource usage info in UI

show the broadcast variables resource usage in UI

![image](https://cloud.githubusercontent.com/assets/678008/4696076/6bb23818-57e5-11e4-83b9-a59ced0d1bc4.png)

Well when the user does sc.broadcast(1), the variable is only created on the driver. Its only when the variable is used that it gets fetched on the executors.  While the driver memory / disk reporting is definitely required, I think it might be okay for it only show up after some stage has run.

@rxin - any thoughts ?