Make Crud listeners and actions easier to test

By providing wrappers for the follow properties and methods Crud should be
a whole lot easier to mock and test:

* `$this->_action()` maps to `$this->_crud->action()`
* `$this->_trigger()` maps to `$this->_crud->_trigger()`
* `$this->_listener()` maps to `$this->_crud->listener()`
* `$this->_session()` maps to `$this->_crud->Session`
* `$this->_controller()` maps to `$this->_controller`
* `$this->_request()` maps to `$this->_request`
* `$this->_model()` maps to `$this->_model`
* `$this->_subject()` maps to `$this->_crud->getSubject()`