Remove unused "entity_manager" option from configuration

`entity_manager` option is not supported in `sonata_doctrine_mongo_db_admin` configuration block.
Merge pull request #107 from whisller/patch-1

Remove unused "entity_manager" option from configuration
Allow multiples document managers in model manager
Update composer.json
Merge pull request #109 from EmmanuelVella/composer.json

Update composer.json
Merge remote-tracking branch 'sonata-project/master'