Currently, spark-shell shares its command history with scala repl.

This fix is simply a modification of the default FileBackedHistory file setting:
https://github.com/scala/scala/blob/master/src/repl/scala/tools/nsc/interpreter/session/FileBackedHistory.scala#L77

Author: Aaron Davidson <aaron@databricks.com>

Closes #267 from aarondav/repl and squashes the following commits:

f9c62d2 [Aaron Davidson] SPARK-1349: spark-shell gets its own command history separate from scala repl