Added shortcut methods for controllers

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#11593)
| Applies to    | 2.6+
| Fixed tickets | #4666 

This commit is associated to https://github.com/symfony/symfony/pull/11593 that adds new shortcut methods to controllers.

If anything is wrong with my commit or this description, please tell me and I will fix it as soon as possible. I'm glad I finally try to help this awesome project.

Same thing as with `redirectToRoute`, I think we should show the "old" (or manual) way of doing this commented-out. This will help users know what's *really* going on and how they might add a flash message if they were not in a controller.