Added shortcut methods for controllers

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#11593)
| Applies to    | 2.6+
| Fixed tickets | 

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.

I think that we should replace all `$this->redirect` with this new `$this->redirectToRoute`. I want to show people the easiest way of doing things. In the `versionadded`, we can say something like:

```
.. versionadded:: 2.6
    The ``redirectToRoute()`` shortcut method was added in Symfony 2.6. The longer way, which
    involves using `generateUrl()` and creating a 
    :class:`Symfony\\Component\\HttpFoundation\\RedirectResponse` object, still works like before.
```