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

[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

Author: Bernardo Gomez Palacio <bernardo.gomezpalacio@gmail.com>

Closes #433 from berngp/feature/SPARK-1522 and squashes the following commits:

2c2e118 [Bernardo Gomez Palacio] [SPARK-1522]: YARN ClientBase throws a NPE if there is no YARN Application specific CP
[SQL] Simple framework for debugging query execution

Only records number of tuples and unique dataTypes output right now...

Example:
```scala
scala> import org.apache.spark.sql.execution.debug._
scala> hql("SELECT value FROM src WHERE key > 10").debug(sparkContext)

Results returned: 489
== Project [value#1:0] ==
Tuples output: 489
 value StringType: {java.lang.String}
== Filter (key#0:1 > 10) ==
Tuples output: 489
 value StringType: {java.lang.String}
 key IntegerType: {java.lang.Integer}
== HiveTableScan [value#1,key#0], (MetastoreRelation default, src, None), None ==
Tuples output: 500
 value StringType: {java.lang.String}
 key IntegerType: {java.lang.Integer}
```

Author: Michael Armbrust <michael@databricks.com>

Closes #1005 from marmbrus/debug and squashes the following commits:

dcc3ca6 [Michael Armbrust] Add comments.
c9dded2 [Michael Armbrust] Simple framework for debugging query execution