Added sum() which gets the sum of values for a given key in a Collection

I think it would be useful to have a simple sum() method on Collections. Very often you have to iterate over a collection of items, and also use the total value of one field somewhere. Of course you can issues a query with sum aggregate but that's not necessary since we already hold all of the information in the collection.

Since there are already min() and max() methods I think it's only logical to have sum() as well :)