[SPARK-1704][SQL] Fully support EXPLAIN commands as SchemaRDD.

This PR attempts to resolve [SPARK-1704](https://issues.apache.org/jira/browse/SPARK-1704) by introducing a physical plan for EXPLAIN commands, which just prints out the debug string (containing various SparkSQL's plans) of the corresponding QueryExecution for the actual query.
seems a little bit strange to have one row per row ... I think the plan should be a single value.