Quaderno API v1 should take selector (currently set to always use a particular ID)

The Stripe API allows one to use any CSS selector they want for the form, but http://js.quaderno.io/v1/ has a number of repeated references to `#payment-form`. A lot of people are already using Stripe successfully with a different form selector, and would like to avoid IDs for various reasons. Being able to provide a selector for the form would be better. 
