PG: Add Support for GroupBy's Cube, Rollup and Grouping Set

[MohammadHabbab]

This Adds support for new syntax added to PG 9.5, namely, CUBE, ROLLUP and GROUPING SET by augmenting PostgreSQL's Visitor with the appropriate unary nodes.

An additional node type GroupingElement has been added as a unary node to allow forcing parenthesis grouping for other syntactic nodes parameters, since parenthesis are significant for CUBE, ROLLUP and GROUPING SET semantics.

[rails-bot]

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[akarzim]

I think the correct semantic is GROUPING SETS (with a final S) regarding to the documentation: https://wiki.postgresql.org/wiki/What%27s_new_in_PostgreSQL_9.5#GROUPING_SETS.2C_CUBE_and_ROLLUP

Or have I misunderstood something ?

[md5]

@akarzim You're correct