Payment

Alright, @wallawe. This is a pretty biggie. This sets the following flow:

- Someone creates a theme
- If they don't have a `stripe_account` associated with themselves, we create one for them
- They then choose the type of payment method they prefer: credit card or bank account
- When they submit this, we create a `Stripe::Recipient` on their behalf, which allows us to pay them

Then...
- Another person buys a theme
- We create a `Stripe::Customer` for them and initiate a `Stripe::Charge` for them
- We then issue a `Stripe::Transfer` to the owner of the theme for the percentage of the theme they earn

**TODO:**

- At the moment this only works for setting a credit card, not bank account
- Also it doesn't appear to work in the view. I have to add the payment method via Stripe's dashboard
- We need to add ourselves as a `Stripe::Recipient` so we can transfer money to us too, as well as maybe the Semantic founders