added after creating to factories

[josephmancuso]

Closes #252

This PR adds the ability to add an after_creating method to the factory class.

You can now do something like this:

def add_order(self, faker):
    return {
        "subtotal": faker.amount(),
        "tax": faker.amount(),
    }

def after_order(self, model, faker):
    model.grand_total = model.subtotal + model.tax

factory.register(User, add_order)
factory.after_creating(User, after_order)

Now the order model will be passed to this after_order callback when you create the order class:

order = factory(Order).create()
order.grand_total #== now set because it was passed to the `after_order` method