[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)

en...I'm not sure... e.g. when the user created sc.broadcast(1), it should be shown on the UI