First pass at creating the plan at Stripe

I'm using an `after_commit` hook to queue a service worker to do the actual interaction with Stripe.  Normally I'm not the biggest fan of `ActiveModel` lifecycle hooks, but for a drop-in gem this seems like the best solution.  It feels heavy to have to tell the developer to be sure to call `Payola.queue(Payola::CreatePlan,...)` in their own code.  Thoughts?
This is where that `safe_constantize` has to happen.