Updated namespace usage.
Changes in JsonView and JsonMiddleware.

Change: Replaced HTTP result numbers with constants defined in JsonView.
Add: Classes comments.
API now fetches single additive properties.
API update: /categories no londer under /additives.
Added /categories route to API.

Additives categories now fetched with a separate route that isn't a part of the general /additives route.
Removed unneeded constructor in model.