Add explicit text/plain template engine

This change allows you to return something like this:

    Results.text().render(object);

The object's `toString()` will be invoked by the `TemplateEngineText` class.

This is more natural than the currently supported syntax:

    Results.text().renderRaw(object.toString());

and fits in better with the other stock return types:

    Results.json().render(object);
    Results.jsonp().render(object);
    Results.html().render(object);
    Results.xml().render(object);

With the current state of *develop* the former syntax will throw an exception because of the unbound template engine for `text/plain`.