Expose properties from registration field so that Rules, Token and EntityMetadataWrapper can access them

In order to properly access settings for a registration field (e.g. in metadata wrappers, rules, etc), there should be a property callback on the field that pulls data from the {registration_entity} table for a given entity. Attached patch setups up property_callback in registration_field_info() and maps the info for all of the columns in the {entity_registration} table.

One drawback, though, is that any additional settings added to registration settings won't be available. Not sure a way around that.