Allow parent to be passed into create methods

[mnussbaum]

First off just want to say thanks, this project has been super helpful to me as I've been testing a DB backed application that features a complex graph of related models.

One feature that would be very helpful is if I could pass an explicit parent into a factory create method. This would let me use a factory as both a subfactory in a chain of related factories and in isolation to create a new instance of a model to add to an existing chain of related models.

Let me know what you think!