Added to_param to Mongoid::Paranoia.

Allows the generation of URLs of deleted models. Makes it easier to
create a list of trashed models (a.k.a Trash view). Helps me with #2226.

Signed-off-by: Mario Uher <uher.mario@gmail.com>
to_param will now return nil again for new paranoid models.

Extention for #2226.