Add setup fees for subscriptions

I need a way to charge a one-time set up fee for subscriptions.

[There are two ways to do this](https://support.stripe.com/questions/metered-subscription-billing):
* Set an ```account_balance``` on the customer at the same time as setting a plan.
* Create an invoice before setting the plan (for itemization on the invoice, would require a name for the charge).

This PR just implements the first. Like coupons, it just passes the setup_fee directly to Stripe (as account_balance).

Thoughts?