[SQL] SPARK-1427 Fix toString for SchemaRDD NativeCommands.

Author: Michael Armbrust <michael@databricks.com>

Closes #343 from marmbrus/toStringFix and squashes the following commits:

37198fe [Michael Armbrust] Fix toString for SchemaRDD NativeCommands.
Merge branch 'master' of https://github.com/apache/spark into SPARK-1413