Add possibility to use a singleton Controller

[decebals]

This PR offers you the possibility to specify a Controller instance for all requests (singleton).

addControllers(ContactsController.class); // one instance for EACH request
// or
addControllers(new ContactsController()); // one instance for ALL requests

See #324 for details.

[coveralls]

Coverage decreased (-0.02%) to 18.994% when pulling e9d6db0 on singleton_controller into 84ea840 on master.

[decebals]

The Controller class plays only a decorative role, it contains only helper methods:

public class Controller {

    public final RouteContext getRouteContext() {
        return RouteDispatcher.getRouteContext();
    }

    public final Request getRequest() {
        return getRouteContext().getRequest();
    }

    public final Response getResponse() {
        return getRouteContext().getResponse();
    }

    @SuppressWarnings("unchecked")
    public <T extends ControllerApplication> T getApplication() {
        return (T) getRouteContext().getApplication();
    }

}

In theory, in the future, we can remove the actual constrain that a controller must extends the Controller class.