A UI for the BlockManager and RDD Stats

It's a WebUI to see how much memory and disk space an RDD and its blocks are taking up. By default started on port 9080. Wanted to get some feedback for the code.

Some notes:
- Added a method to name an RDD. The names are stored locally in the SparkContext.
- Added an Map[Int, RDD] to the SparkContext, because I couldn't find an easy way to get to the RDD properties (like StorageLevel) given an RDD ID.