Support the payment protocol

[ryanxcharles]

The payment protocol is a much better way to pay with bitcoin and copy+pasting addresses. It supports authentication of the merchant, sending a tx to the merchant for them to broadcast, and sending refund addresses in case anything goes wrong. It should significantly reduce overpayments and underpayments and should be all-around more pleasant for consumers and merchants.

Bitcore can be used by both wallets and merchant software. We should support the payment protocol on both sides of the use spectrum. See BIP 70, 71, 72, and 73:

https://github.com/bitcoin/bips/blob/master/bip-0070.mediawiki
https://github.com/bitcoin/bips/blob/master/bip-0071.mediawiki
https://github.com/bitcoin/bips/blob/master/bip-0072.mediawiki
https://github.com/bitcoin/bips/blob/master/bip-0073.mediawiki

Bitcoin Core and bitcoinj already support the payment protocol. We should test against both of those.

[ryanxcharles]

Relevant PRs:

• Add payment protocol example #456
• WIP: BIP-70 x509 signing and verification #435
• Support for "SIN" pki_type in BIP70 #405
• add BIP70 protobuf features in new PayPro lib file #401

[martindale]

+1, awesome. Great work, @ryanxcharles!