- Add ephemeral_public_key and requires_payment_message fields to EncryptedPaymentRequest + updated descriptions
- Update EncryptedPayment and EncryptedPaymentACK message descriptions to use ECDH-derived key for signature instead of each side's public key
- Slim down message content-types
- Add EncryptedPayment and EncryptedPaymentACK creation detail steps
- Add updated paymentrequest.proto to bip-ir/ directory
- Add additional flow diagrams for various mobile-to-mobile / Store & Forward scenarios
