Loads Registry::Rule from Engine::Object.
Updates sprite sheet json to allow extensibility.

Adds "frames" field that contains each image within the sheet. Adds
"author" and "license" fields that contain strings for the attribution.

TODO: Consider distributed identities.
Adds "type" field to sprite sheet json.

Does not enforce it yet.
Adds documentation to World::Scene for actorCount and mapCount functions.
Moves Agent namespace to Rules namespace and consolidates.

All Rules are just listed under the rules namespace. It is too confusing
to have to consider what "type" of rule each is, so they can all attach
collide/act/updaters as they will and all are just considered rules.

They will self-describe in metadata later.