Add the Admin extension that enables editing HTML attributes

Contains an interface that encapsulates the attributes related methods for the menuNode.
Relies on burgov/key-value-form-bundle for the admin.
This bundle also has a use_sonata_admin config. We could throw an exception if that is set to true and SonataDoctrinePHPCRAdminBundle is not installed, currently the extension just bails out without loading admin.xml

https://github.com/mkoosej/MenuBundle/blob/master/DependencyInjection/CmfMenuExtension.php#L106