[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)

I just looked at the possible solution to update broadcast var info upon the stage finished (just attempted to implement this in a single commit so that I can easily revert to shorten the RTT), the problem is, how to associate a broadcast variable with the stage....I didn't find a very reasonable solution on this, because for RDD, we need that information to create a stage object, so everything works straightforwardly...for broadcast...I'm not sure if we can really do that....maybe I missed something 

@shivaram @rxin ?