[SPARK-8607] SparkR -- jars not being added to application classpath correctly

Add `getStaticClass` method in SparkR's `RBackendHandler`

This is a fix for the problem referenced in [SPARK-5185](https://issues.apache.org/jira/browse/SPARK-5185).

cc shivaram

Author: cafreeman <cfreeman@alteryx.com>

Closes #7001 from cafreeman/branch-1.4 and squashes the following commits:

8f81194 [cafreeman] Add missing license
31aedcf [cafreeman] Refactor test to call an external R script
2c22073 [cafreeman] Merge branch 'branch-1.4' of github.com:apache/spark into branch-1.4
0bea809 [cafreeman] Fixed relative path issue and added smaller JAR
ee25e60 [cafreeman] Merge branch 'branch-1.4' of github.com:apache/spark into branch-1.4
9a5c362 [cafreeman] test for including JAR when launching sparkContext
9101223 [cafreeman] Merge branch 'branch-1.4' of github.com:apache/spark into branch-1.4
5a80844 [cafreeman] Fix style nits
7c6bd0c [cafreeman] [SPARK-8607] SparkR