API additions for payment provider integration

Hi,

I'm the maintainer of the Commerce SP PayPal module, which integrates the PayPal payment system with subscription products. The code is based on the 2.x branch and requires the patch from #1522018: Missing feature fields causes rules data set errors. (comment #37). The Commerce SP PayPal module contains a few API helper functions that ease the programmatically handling of recurrent subscriptions.

Now we're planing to integrate Paymill as another payment provider for subscription products. For this integration we'll require the same API helper functions so it would make sense to move them directly into the SP module.
To name some examples:

• commerce_sp_paypal_signup -> Sets skip automatic expiration field + lock management
• commerce_sp_paypal_stop_subscription -> Unsets skip automatic expiration field
• commerce_sp_paypal_clone_order -> Clones an order when the subscription is renewed

There hasn't been any progress for this module the last year. I'm offering my help in committing the most important patches for the 2.x branch, like #1522018: Missing feature fields causes rules data set errors., and adding some API functions for payment provider integration.

Matthias