Do not create symlink for local add file. Instead, copy the file.

This prevents Spark from changing the original file's permission, and
also allow add file to work on non-posix operating systems.
Fixed the flaky unpersist test in RDDSuite.
More block manager cleanup.

Implemented a removeRdd method in BlockManager, and use that to
implement RDD.unpersist. Previously, unpersist needs to send B akka
messages, where B = number of blocks. Now unpersist only needs to send W
akka messages, where W = the number of workers.
Incorporated Charles' feedback to put rdd metadata removal in
BlockManagerMasterActor.
Merge pull request #636 from rxin/unpersist

Unpersist More block manager cleanup.