make get_parent/child_types() able to return all possible parents or children

In the issue to create a 'my foo' block (http://drupal.org/node/103834) I needed the functions nodefamily_get_child_types() and function nodefamily_get_parent_types() to be a tad more flexible: they should optionally return all types.

This patch achieves that, buy making the $type parameter optional: if it is not passed along, the API will return not just the children/parents for that $type, but all possible children/parents.