[WIP] add translation extension

To continue with our discusion in ForumPHP here is my PR to add translation support in sonata.

**Admin**
- add alterObject methods to bind translations
- add getPersistentParameters in the extension
- update admin and interface to fit to this new methods

**new extension**
- add AbstractTranslatableAdminExtension to provide locale support


**todo**
Add locale switcher template

**Question**
- how do you want me to handle parameters for default locale and the list of locales possible
For the moment I added a default_locale + locales parameter but in the locale switcher template I have to get the locales list

I used to do this by adding a global variable to twig but this means that every body will have to add it in their configuration.

Maybe I can add this to the bundle configuration and store it in the Admin class, want do you prefer ?