Added support for multiple subscriptions

This implementation of Stripe's new multiple subscriptions features works for me.

Note that I had to upgrade the required stripe gem version to 1.10.1 because before that there wasn't a `Subscription` object in the stripe gem. Also note that I had to take out the old `update_subscription` and `cancel_subscription` methods on the `Customer` object. Pushing this out's going to break the gem for people who haven't upgraded to the latest version of the Stripe API, which is unfortunate.