Proof of Payment URI scheme.

Proposed a link format, similar to bitcoin: payment links, for asking a wallet to produce a Proof of Payment.

This is a companion to BIP 120. Whoever wants the proof creates a btcpop: link, which can be shown as a QR code or sent over NFC to the payer's wallet. The wallet then sends its proof to the destination named in the link, such as a web address.