[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:
* ScalaBuild now points to Scalatest 2.1.3
* ScalaBuild project "root" renamed as "spark"
* Test Suite for ClientBase added
* Fixes for scalastyle in other yarn files.

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

Author      : bernardo.gomezpalacio@gmail.com
Testing     : SPARK_HADOOP_VERSION=2.3.0 SPARK_YARN=true ./sbt/sbt test