Allow a model proxy to not proxy any methods.

Initializer now defaults to an empty list for the method proxy list.
Add links into a user model.

This is so that the main app can have users more easily as it is a common
requirement.
Set the user_id of a new run to be the current user.

This uses a callback defined in the initializer to use whatever method the main
application uses to determine the current user.
Add tests for user integration.

A User model is added to the dummy app for this.
Force overflow value encoding to UTF-8 on save.
Fix large value overflow into file.

Now behaves correctly with changing the file blanking out the value and both
file and values set at the same time on create or update prevented.