- Add Subscription API (John Ku, #3)
  - Add Invoices API (Chris Winn, #6)
  - Fix a bug where callback could be called twice, if the callback()
    threw an error itself (Peteris Krumins)
  - Fix bug in tokens.retrieve API (Xavi)
  - Change documentation links (Stripe changed their URL structure)
  - Make tests pass again (error in callback is null instead of 0 if all is well)
  - Amount in stripe.charges.refund is optional (Branko Vukelic)
  - Various documentation fixes (Xavi)
  - Only require node 0.4.0