Correct prefix when PSR-4 bundle

Recently the BazingaOAuthBundle moved to a PSR-4 folder structure and it seems that the PropelBundle doesn't handle this structure correctly. Assuming I have a project with both the BazingaOAuthBundle and the FOSUserBundle and I am trying to generate the correct model classes;

**FOSUserBundle**
_$bundle->getPath()_ => /vendor/friendsofsymfony/user-bundle/FOS/UserBundle
_$bundle->getNamespace()_ => FOS\UserBundle
_$prefix_ => vendor.friendsofsymfony.user-bundle.

**BazingaOAuthBundle**
_$bundle->getPath()_ => /vendor/willdurand/oauth-server-bundle
_$bundle->getNamespace()_ => Bazinga\OAuthServerBundle
_$prefix_ => vendor.

This pull request fixes the generation of the prefix. I am still looking at the class generation of Propel itself. This should be `/vendor/willdurand/oauth-server-bundle/Propel` but is `/vendor/willdurand/oauth-server-bundle/Bazinga/OAuthServerBundle/Propel`.