[SPARK-2219][SQL] Added support for the "add jar" command

Adds logical and physical command classes for the "add jar" command.

Note that this PR conflicts with and should be merged after #2215.
The `ThreadLocal` version was added in #993, and I see this as the more recommended manner to pass `SQLContext`/`HiveContext` since it saves boilerplate code, @marmbrus is it?