Core: implement @groupBy directive

[binier]

Implement Dgraph's @groupBy directive.

blocked by: #4 since only aggregations can be run inside grouped edge.

[snipebin]

@binier #4 is closed - is this still blocked?

[binier]

@snipebin no longer, since aggregations are implemented. Should be pretty easy to implement, just haven't had any time to dedicate to it.