Implement Aggregation

[heynemann]

Allow users to aggregate data using the new Aggregation Pipeline(http://docs.mongodb.org/manual/core/aggregation-pipeline/):

class Product(Document):
    type = StringField()
    location = StringField()
    price = FloatField()
    stocked = IntField()
    is_active = BooleanField()

Document.objects.filter(is_active=True).order_by("type").aggregate(
    group_by=[Document.type, Document.location],
    price=Aggregation.add_to_set(Document.price),
    stocked=Aggregation.add_to_set(Document.stocked)
)

The types of aggregation are in the following tickets:

• [ ] Implement add_to_set aggregation - #47
• [ ] Implement first aggregation - #48
• [ ] Implement last aggregation - #49
• [ ] Implement max aggregation - #50
• [ ] Implement min aggregation - #51
• [x] Implement avg aggregation - #52
• [ ] Implement push aggregation - #53
• [ ] Implement sum aggregation - #54
• [ ] Implement count aggregation - #55