Provide mechanism to override default view

[james]

A render() function, or a way to call output() to override the default (views/controller/action.php) view.

Basically, in an action:

• If render() (or whatever) is called, display the output of render() immediately.
• If render() was called by the end of the action, the default Controller::output() method is not called.
• If render() was not called, fall back to the default method now in place.

May need to make it easier to render the default header/footer templates, eg: render_(header|footer)(). Though render('application/header.php') should work.