stripe integration

This is currently functional, but not complete. Please note:

* Video price are now in cents to match Stripe. The migration will clobber existing prices, so sorry about that. Also, we'll need to divide by 100 when we show prices in the views (probably with a helper).

* The auth hash we receive when people sign in with an external service is now serialized and stored on the `Auth` model. This is just so we have that info saved and can refer to it later if we want.

* I'm using stripe checkout to collect payment info on the video page, but I still need to add a button that skips that step for users that already have payment info stored. Also, you guys might want to switch to a [custom button][0] for the checkout stuff.

* I feel kinda strange about the fact that nothing in stripe checkout says that we are storing their information for later use. It's not a big deal, but we should probably note that somehow along the way, potentially with an opt out option if they don't want their info saved.

* There is no management of multiple cards, expiring cards, setting default cards, etc yet.

* Still need to add a local `Purchase` model to track everything in our database. The Stripe API and dashboards are awesome, but we obviously want to keep some of that info locally for easy querying.

I'll probably wait until I get everything a bit more polished before merging, but feel free to check out the code and let me know if you have any questions. 

[0]: https://stripe.com/docs/checkout