Added hosted_page_url and hosted_update_payment_page_url methods to Subscription

These changes will allow you to generate the hosted "update_payment" page URLs from a Subscription instance. It generates the URL with a 10 character SHA1 token as per documentation: http://docs.chargify.com/hosted-page-integration#hosted-page-urls
We have added this functionality in commit 598298d5. Would you mind removing `migration_preview` and so that we can go with the implementation that already exists?