Add schema registry with versioning

[mikesparr]

If we are going to allow dynamic nodes in graph, and dynamic document types in #53 then we need ability to define and manage schemas and versioning. (go add type version field for #53)

Proposed schema

• id: string (enum of ObjectType)
• version: number
• structure: any
• published: string (ISO_8601)
• archived: boolean

UNIQUE index on id, version fields