[SPARK-1522]: YARN ClientBase throws a NPE if there is no YARN Application specific CP

The current implementation of ClientBase.getDefaultYarnApplicationClasspath inspects
the MRJobConfig class for the field DEFAULT_YARN_APPLICATION_CLASSPATH when it should
be really looking into YarnConfiguration. If the Application Configuration has no
yarn.application.classpath defined a NPE exception will be thrown.

Additional Changes include:
* Test Suite for ClientBase added
* Coding Style:
    * [Spark Style Guidelines](https://cwiki.apache.org/confluence/display/SPARK/Spark+Code+Style+Guide)
    * [Scala Official Style Guidelines](http://docs.scala-lang.org/style/)
    * [Scalariform](https://github.com/mdr/scalariform)
* Code refactoring and cleanup per review by andrewor14

[ticket: SPARK-1522] : https://issues.apache.org/jira/browse/SPARK-1522

Author      : berngp
Reviewer    : andrewor14, tgravescs
Testing     : ?