Add group_by_grouping_sets query method

[leboshi]

Implements GROUP BY GROUPING SETS. (Haven't done CUBE or ROLLUP yet.)

Example:

users = User.group_by_grouping_sets([], [:name], [:name, :age]).select(:name, :age, 'count(*)')

generates:

SELECT name, age, count(*) FROM users GROUP BY GROUPING SETS ( (), (name), (name, age) )

Accepts arrays of strings, symbols, and Arel nodes, as well as any arbitrary mix of them.