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)

For more information, you can have a look at this PR: https://github.com/sonata-project/SonataFormatterBundle/pull/46

Thank you @dunglas for your work.