Add compatibility support for keyf in group command

Hi,
I'm new to mongojs and relatively new to mongo at all. In the mongo shell I issued a group command as documented on http://docs.mongodb.org/manual/reference/method/db.collection.group/ with a function creating the grouping key. Therefore I used the **keyf** parameter and passed in a function. Back in my node.js project I copy and pasted the group command from the mongo shell and it did not work since the **keyf** parameter is not used by mongojs. 

Since mongojs should be compatible with mongo, I created a small compatibility patch to support both **key** and **keyf**.

Cheers,
Ben