sqlgen: Adding GROUP BY clause support.
sqlgen: Removing unrequired field.
Adding interface for grouping results by column (Group()).
PostgreSQL: Adding support for db.Result.Group().
MySQL: Adding support for db.Result.Group().
Fixing comment formatting for db.Result.Group().
SQLite: Adding support for db.Result.Group().
QL: Adding support for db.Result.Group().
MongoDB: Adding placeholders for group by support. Currently not implemented.