Charge for private repos

https://trello.com/c/Rq3YdC3K/174-charge-for-hound
@croaky Stripe Checkout worked great! The only piece missing, which we will put off for now, is using an existing card. The user will be prompted each time they subscribe, but not for long.

![screenshot 2014-08-08 09 36 46](https://cloud.githubusercontent.com/assets/154463/3859580/3e6bfdfc-1f1a-11e4-9595-b615d6162f14.png)