Added ability to change Subscription quantity

The creation method defaults to `quantity` of 1, but a different `quantity` may be passed. It's sent the same as the coupon codes (shows up in SubscriptionsController as `params[:quantity]`). There is now a `change_quantity` route that acts similarly to `change_plan`, and calls a `quantity_changed` event. 

There should probably be a way to update the quantity when `change_plan` is executed, because it defaults 1 (which is what Stripe defaults it to as well if you don't pass in a `quantity` when changing plans, meaning the quantity you had on the old plan won't transfer to the new one unless you set it explicitly).