Investigate EIP 1337 for subscriptions

[cryppadotta]

EIP 1337 is standard for recurring payments / subscriptions and it is probably a good idea for the next version of dotta-license.

See:

• EIP 1337 -- Subscriptions on the blockchain. ethereum/EIPs#1337 and
• https://github.com/austintgriffith/tokensubscription.com/tree/13ced407c2709e99fbe7838a84a9d53f855b40bc