Add factorys for ModelLoaders for runtime config

Defaults don't change, but allows you to
dynamically add new loaders for models or replace
the defaults.
Move volley classes to subpackage

Allows us to almost completely exclude volley,
including our various adapter classes
Add a model cache

Allows generalized caching from caches stored in
factories for expensive operations (like
parsing a string to a URL).
Error handling with no registered URL factory