Init command

this adds an Initializer interface and the ``doctrine_phpcr.initializer`` tag along with a command doctrine:phpcr:init to the bundle, to allow all bundles to register custom node types and create base paths. this is similar to the doctrine:schema:create command of the orm.
should we call the command doctrine:phpcr:workspace:init or doctrine:phpcr:repository:init ? its kind of a mix between the two.