Add spec for revision handling

Following #87 (closed), there needs to be a core specification for handling conflicting data and access to prior revisions in eventually consistent architectures.

All UIs (regardless of whether the backend implements such features) will need to implement these interfaces in order to be considered "fully compatible" with the spec. This implementor burden should be kept as minimal as possible; ergo the smallest number of additional fields and simplest application logic should be favoured.