BIP75 Update: Versioning, Cancellation, Unknown Message Type, New PKI_TYPEs, Status Codes, Motivation

Added:

• Versioning
• Cancellation
• Add UNKNOWN_TYPE to ProtocolMessageType enum in paymentrequest.proto and BIP75 text (based on suggestions from http://androiddevblog.com/protocol-buffers-pitfall-adding-enum-values/)

Updated:

• Additional pki_type values
• Updated status_code values
• Update BIP75 Motivation and use cases to provide more color around reasoning and use of BIP75

NOTE: The BIP75 language no longer contains a description of the KYC compliance use case, as it is a single, very specific use-case that does not have any bearing on the technical specifications herein. BIP75 extends the original BIP70 Payment Protocol to become a two-way, encrypted messaging process, which can be used for a variety of reasons one of which is regulatory compliance.