[SPARK-2663] [SQL] Support the Grouping Set

Add support for `GROUPING SETS`, `ROLLUP`, `CUBE` and the the virtual column `GROUPING__ID`.

More details on how to use the `GROUPING SETS" can be found at: https://cwiki.apache.org/confluence/display/Hive/Enhanced+Aggregation,+Cube,+Grouping+and+Rollup
https://issues.apache.org/jira/secure/attachment/12676811/grouping_set.pdf

The generic idea of the implementations are :
1 Replace the `ROLLUP`, `CUBE` with `GROUPING SETS`
2 Explode each of the input row, and then feed them to `Aggregate` 
  * Each grouping set are represented as the bit mask for the `GroupBy Expression List`, for each bit, `1` means the expression is selected, otherwise `0` (left is the lower bit, and right is the higher bit in the `GroupBy Expression List`)
  * Several of projections are constructed according to the grouping sets, and within each projection(Seq[Expression), we replace those expressions with `Literal(null)` if it's not selected in the grouping set (based on the bit mask)
  * Output Schema of `Explode` is `child.output :+ grouping__id`
  * GroupBy Expressions of `Aggregate` is `GroupBy Expression List :+ grouping__id`
  * Keep the `Aggregation expressions` the same for the `Aggregate`

The expressions substitutions happen in Logic Plan analyzing, so we will benefit from the Logical Plan optimization (e.g. expression constant folding, and map side aggregation etc.), Only an `Explosive` operator added for Physical Plan, which will explode the rows according the pre-set projections.

A known issue will be done in the follow up PR:
* Optimization `ColumnPruning` is not supported yet for `Explosive` node.


If this is only used in analysis then lets move it to `analysis.scala` and model it after the other unresolved expression types.

Can you also clarify how it is used in the scala doc.