[SPARK-8992][SQL] Add pivot to dataframe api

This adds a pivot method to the dataframe api.

Following the lead of cube and rollup this adds a Pivot operator that is translated into an Aggregate by the analyzer.

Currently the syntax is like:
~~courseSales.pivot(Seq($"year"), $"course", Seq("dotNET", "Java"), sum($"earnings"))~~

~~Would we be interested in the following syntax also/alternatively? and~~

    courseSales.groupBy($"year").pivot($"course", "dotNET", "Java").agg(sum($"earnings"))
    //or
    courseSales.groupBy($"year").pivot($"course").agg(sum($"earnings"))

Later we can add it to `SQLParser`, but as Hive doesn't support it we cant add it there, right?

~~Also what would be the suggested Java friendly method signature for this?~~

Author: Andrew Ray <ray.andrew@gmail.com>

Closes #7841 from aray/sql-pivot.