[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)

From what I can see RDD blocks are updated at the end of a stage ? That should be fine for broadcast blocks too if that is the case -- I am just not sure we need a new listener event, filter etc for just this use case.