Checkout serializer missing allow_blank=True

[thenewguy]

Probably related to #29 and #30 but mentioning issue so it is tracked.

It would be helpful for situations where a customer genuinely does not have an email address to allow checkout without one.

The Order model does not require an email address but the Checkout serializer does.

Simplest workaround I've come up with for the moment is to submit with an email placeholder and then remove it with a pre_save signal.