[SPARK-2710] [SQL] Build SchemaRDD from a JdbcRDD with MetaData

SPARK-2710 Build SchemaRDD from a JdbcRDD with MetaData

and a small bug fix on JdbcRDD, line 109
it seems conn will never be closed


For the 1.2 release we are going to be focusing on adding more external datasources.  As part of this we are trying to change the way we add them to avoid SQLContext getting to large.  What do you think about adding an object, `org.apache.spark.sql.jdbc.JDBC` that has these methods instead of adding them here?