Rename 'resource_uri' to '_uri' in JSON objects

"_uri" is more compact. The initial underscore clearly highlights it as metadata and this pattern should be used with any new metadata to be added. There is the possibility of adding "_id" later on without risking a clash with fields named "id" or similar.
Rename "resource_uri" to "_uri" in comments
Refactor NestedProxy types to ResourceRoute and refactor schema

- Schema now split between API/Resource/Field levels
- ResourceMethod now ResourceAction
- SchemaParser to replace flask_restful's RequestParser