Message Object Schema

[micahalcorn]

As recommended by @crazybuster (and @franckc offline), we should prescribe (and probably version) a schema for the message object and move the logic into the schemas directory. It is currently bare-bones and lives in the messaging resource. The schema evolves a bit in my arbitration-test branch, but we need to decide how to structure data URIs and other things that should not simply be rendered as content strings. It's worth noting that, as is the case with our other schemas, any other developer can store data in a non-conforming format since the storage is decentralized.