Add support for weekly plans.

Currently, stripe-rails only accepts values of "month" and "year" for plan intervals. However, [the Stripe API allows "week" as an option](https://stripe.com/docs/api/ruby#plan_object) to create plans that bill each week. This pull request addresses the issue and adds some tests to make sure "week", "month", and "year" all work while still rejecting arbitrary values for the plan interval.

Thanks for the work on this - it definitely helps to not have to recreate the wheel!