More Subscriptions WIP

This brings `Payola::Subscription` a little more in line with `Payola::Sale` in terms of the processing flow.  The whole flow is working to create a subscription, process it, check on status, and submit the custom form once the subscription is activated.  The `aasm_state` at the end of this flow is currently called `finished`.  It'll probably be worth considering having some different state names for subscriptions.  Maybe `:pending => :processing => :active => :cancelled`?

BTW, what's your preferred method for random discussions around payola development?  Should I just create new issues if I have a big picture question for you about subscription stuff?  **updated:** On second thought, maybe the initial "Subscriptions" issue is a good place?