Create Subscription

This removes `stripe_id` from `User` and creates a `Subscription`. A wise man [once said](https://twitter.com/r00k/status/520644915018346499):

![screen shot 2014-10-10 at 1 18 46 pm](https://cloud.githubusercontent.com/assets/65323/4598583/aef7fa68-50ba-11e4-97c5-e4320c1ebcec.png)