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

WIP, finished the data channel, need to reorganize the pages

![image](https://cloud.githubusercontent.com/assets/678008/4696076/6bb23818-57e5-11e4-83b9-a59ced0d1bc4.png)

Would this send a BlockStatus for each broadcast variable on a heartbeat ? If we have hundreds or thousands of broadcast variables I wonder if the message size will become huge. Could we send deltas somehow ?