Add admin generate command for doctrine

I have added a generate command which let you generate admin files and config for an 
entity.

php app/console sonata:admin:generate-doctrine MyBundle:Entity MyAdminBundle

This command will generate Admin, Controller and config file in MyAdminBundle from entity MyBundle:Entity

I'm planning to add a interactive mode (like bundle generate command for symfony). Currently it's generated all fields we can excepted primary key. It also add "required => true" for non nullable field in form and it's only based on doctrine.