Merge pull request #8 from gmacdougall/one-user-per-user

Promotion Rule: One Use Per User
Adds 'current order' API endpoint

This enables an API client to load a user's latest order. At Bonobos, we
use this to load the user's last active cart.

The find_current_order method is broken out to a separate method to
enable gems like spree-multi-store to customize its behavior.

Closes #5069