Allow any identifier in aggregation group operator

Actually the aggregation `GroupField` and `GroupMulti` are not giving us the full power of mongodb aggregation. Indeed, mongodb does not enforce that we group on document fields. From mongodb documentation:

> Every group expression must specify an _id field. You may specify the _id field as a dotted field path reference, a document with multiple fields enclosed in braces (i.e. { and }), or a constant value.

Source : http://docs.mongodb.org/manual/reference/aggregation/group/#_S_group

This patch allows the library user to group on an arbitrary identifier, including a constant value.