Fix serialization persistence problem. Fixes #35

Serialization was not working mainly due to the following reasons:
- If an object extends ScriptableObject, it must be in a seperate file
  by itself; Assignment and Evaluator classes have been extracted to the
  Operators folder.
- A ScriptableObject must mark all private fields that need to be
  serialized with [SerializeField].

RenPyScriptAssets should be serialized and persisted between sessions
correctly now.