Standard proposal draft (2nd iteration, Stef & Luka)

[lknix]

This is a second iteration of the proposal. Interface has been divided into:

1. Read interface
2. Write interface
3. Metadata interface
4. Interface for processor support

Changelog:

• support for arbitrary payment assets
• support for arbitrary large number of subscriptions (provider's side)
• support all commonly used subscription models (pre-paid, post-paid, limited lifetime, fixed/varying payment amounts, discounted/trial periods)
• creating a subscription is now out of scope of the standard
• added rationale section to explain reasons behind design decisions
• etc.