[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.

Author: Cheng Lian <lian.cs.zju@gmail.com>

Closes #2242 from liancheng/add-jar and squashes the following commits:

e43a2f1 [Cheng Lian] Updates AddJar according to conventions introduced in #2215
b99107f [Cheng Lian] Added test case for ADD JAR command
095b2c7 [Cheng Lian] Also forward ADD JAR command to Hive
9be031b [Cheng Lian] Trims Jar path string
8195056 [Cheng Lian] Added support for the "add jar" command