handle relations in show mode

fix #306

porting the configuration for show templates from the orm admin.
The value for `getMappingType` is set in the [FieldDescription](https://github.com/sonata-project/SonataDoctrineORMAdminBundle/blob/master/Admin/FieldDescription.php) class, see the methods `setAssociationMapping` and `setFieldMapping`.

ClassMetadata constants are numeric values, so there have to be some swtich or if/else conditions to convert it to string. And yes: it can be improved to use some default templates from the configuration.