Bad aggregation-cursor docs

The example snippet for MotorCollection.aggregate with a cursor is missing a "yield" statement:

http://motor.readthedocs.org/en/stable/api/motor_collection.html#motor.MotorCollection.aggregate

Additionally, aggregation is becoming prominent enough it should be included in the tutorial and / or examples sections.

Reported here:

http://stackoverflow.com/questions/24127427/python-motor-offers-fetch-next-exception-on-aggregation