Add CKEditor browsing and uploading medias feature

I've integrated the way to browse and upload medias in ``SonataMediaBundle`` used by the following bundle: https://github.com/coopTilleuls/CoopTilleulsCKEditorSonataMediaBundle.

For more information, see issue: https://github.com/coopTilleuls/CoopTilleulsCKEditorSonataMediaBundle/issues/1

![capture decran 2014-01-29 a 12 28 07](https://f.cloud.github.com/assets/103900/2028922/83d1ea38-88d8-11e3-87af-3514e4c0a462.png)

I've added a new documentation entry named "Use CKEditor to select medias in SonataMediaBundle" in this Formatter bundle to explain how to configure.

Have you got some comments?

Thank you for your work @dunglas

TODO:

 - [x] waiting @dunglas feedback on the header issue
 - [x] waiting @egeloen if CKEditorBundle can be refactored a bit to avoid the scope ``request``
 - [ ] waiting for https://github.com/egeloen/IvoryCKEditorBundle/pull/85