Update installation.rst

Add undocumented configuration for defining custom classes instead of using the default Application/Sonata/MediaBundle
Update SonataMediaBundle.fr.xliff

add some missing french translations
Merge pull request #321 from grizzlylab/master

Add some missing french translations
Merge pull request #313 from Zertz/patch-1

Update installation.rst
Merge branch 'sonata/master'