Specifications for invoice based payments

[fedsten]

The current Kaleidoscope implementation uses a sendtoaddress payment method, which is basically the standard also in Bitcoin wallets. However, since we are building a new protocol, it could be a good opportunity to encourage the usage of invoices, that have the advantage of mitigating human errors with the amount or asset_id, and facilitate the transmission of other relevant information from the payee to the payer (such as the commitment scheme preference, see #82).

For this scope, it would be useful to formalise an invoicing standard in the protocol specifications.