ADDED: Notion of _application_.  Predicates pengine_application/1 and
option application(Application) to pengine_create/1.
CLEANUP: merged child/1 and named_child/2.
Added test for pengine_self/1
ADDED: pengine_name/2
MODIFIED: terminate pengine_event_loop/2 if an error is raised by a
pengine that is not handled.
MODIFIED: Use child/2 to keep track of the known pengines.