v0.10.0 - payment links

Added

• Payment links - createPaymentLink(), paymentLinkStatus(), collectPaymentLink(),
  stopPaymentLink() and, for custom checkouts, openPaymentLink() and subscribePaymentLink(). Invoices paid once, fixed
  prices paid many times, and subscriptions P2Flux collects for you - no server needed.
  (https://p2flux.com/docs/payment-links.html)
• checkoutLink('link' | 'links', $token) for the buyer's link and the merchant's overview.
• Error codes INVALID_LINK, LINK_EXPIRED, LINK_UNAVAILABLE, ALREADY_SUBSCRIBED.