This adds two new class methods for CounterField: increment_many, and

decrement_many. These can be used for incrementing or decrementing many

fields at once on an instance of a model.
It also makes two changes to saving field values.
An initializer can now return None, meaning that CounterField won't set

the field and save the object. That's intended for when an initializer

will be handling setting the field itself, perhaps through

increment_many() or update().
When saving the field, CounterField now passes update_fields to save(),

ensuring we only save those affected fields and don't unintentionally

save other aspects of the model.

Tested along with a change on Review Board that makes extensive use

of these functions.