Renamed storage levels to something cleaner; fixes #223.

core/src/main/scala/spark/KryoSerializer.scala

core/src/main/scala/spark/RDD.scala

.../src/main/scala/spark/broadcast/BitTorrentBroadcast.scala

core/src/main/scala/spark/broadcast/HttpBroadcast.scala

core/src/main/scala/spark/broadcast/TreeBroadcast.scala

core/src/main/scala/spark/storage/BlockMessage.scala

core/src/main/scala/spark/storage/BlockMessageArray.scala

core/src/main/scala/spark/storage/StorageLevel.scala

core/src/test/scala/spark/storage/BlockManagerSuite.scala

docs/scala-programming-guide.md