pick up from the cleanup done by @Clindbergh

continue the PR started in #616
some guesswork here. the code is https://github.com/helios-ag/FMElfinderBundle/blob/master/DependencyInjection/Configuration.php#L30 and https://github.com/helios-ag/FMElfinderBundle/blob/master/Configuration/ElFinderConfigurationReader.php