[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
c6e041d171