Accompany rule triggers with drupal_alter

It can be annoying putting stuff in rules, that aren't supposed to be accessible trough ui after all.
In commerce, most rule triggers are combined with a hook. However, in commerce_coupon, some does not.

Proposing adding two new hooks,
- hook_commerce_coupon_redeem
- hook_commerce_coupon_validate

This gives custom code full control over the workflow.