Feature/extended group by

[ostafen]

Improve support of order by and group by clauses:

• Support order by on any number of fields (with mixed directions (ASC/DESC);
• Support order by on aggregations;
• Support group by without the need to specify an order by clause.

[coveralls]

coverage: 89.487% (+0.07%) from 89.419%
when pulling 533b36b on ostafen:feature/extended-group-by
into 0c8204a on codenotary:master.