Handle 3rd-party plugins that expect a modelClass for all controllers

The CRUD controller currently expects this sort of authentication logic to exist within a Controller, not a plugin Controller that has no models. Forcing `$this->modelClass = null` in the `__construct()` is the only way to do this.
Merge pull request #32 from josegonzalez/patch-1

Handle 3rd-party plugins that expect a modelClass for all controllers
Merge remote-tracking branch 'upstream/master'