Remove custom serializers for parent classes (_ptr_id).

This was fixed in DRF 2.2.4:

https://github.com/tomchristie/django-rest-framework/pull/723
Make clients set a payment method before generating the payment url.
Change Order to checkout after payment is made.
Change Order and Payments relationship to ManyToMany.
Use local function for generating the voucher code.
Use jQuery directly to work-around problem with ember-data.
Merge pull request #254 from benkonrath/i-really-wanna-pay

I really wanna pay