[SPARK-6966][SQL] Use correct ClassLoader for JDBC Driver

Otherwise we cannot add jars with drivers after the fact.

Author: Michael Armbrust <michael@databricks.com>

Closes #5543 from marmbrus/jdbcClassloader and squashes the following commits:

d9930f3 [Michael Armbrust] fix imports
73d0614 [Michael Armbrust] [SPARK-6966][SQL] Use correct ClassLoader for JDBC Driver