[api] Add ability to assign user_id through Orders#update

Hi, my name is associate_user. You might remember me from smash comedies like 'CheckoutsController'.
[api] Return billing address in user data

Useful for when you want to find a user by their email address, and have their address information display on a page

Like this one http://cl.ly/image/3S1P0W2g0W42
Add shipments API back under orders
CreditCard#expiry= used to assume the incoming format was that which was enforced by Frontend's JS.

No longer true.

Fixes #4429
Fixes #4653