GROUP BY clause support

[nobrin]

For aggrigation, QuerySet.group_by() will be implemented.

[dz1984]

It's a great idea. I also need it. :+1:

[nobrin]

Simple GROUP BY clause is considerable. The simple Team.group_by("firstname", "familyname") will be equivalent to SELECT firstname, familyname FROM team GROUP BY firstname, familyname. It returns iterator or list of tuples.