metadata type for non-property mappings

(this is a follow up of https://github.com/sonata-project/SonataDoctrinePhpcrAdminBundle/issues/56)

ClassMetadata::mapField does not automatically assign a type except for the id field. we should add them here to have type information for things like nodename so for example sonata admin (or the symfony form builder?) can use that information to decide on the widget to use.

open question: is there a natural type mapping for children/parent? should we use MANY_TO_ONE for parent and ONE_TO_MANY for children? or is that a problem as its not arbitrary relations, should we add new types for this?