Fixed a bug where defining a pk on an instance would cause it not to be committed to the DB.
Cleanup in the model factory's handling of defined collections.

Re #46
Collections now sort their in ascending order of their primary key.
Queries methods now accept another argument, an array of strings, which will order the results. Uses Django's style of denoting ascending/descending order.

Re #46
Re #17
When loading data without primary keys, primary keys will be provided.