enable dead-end detection, at last

Link: https://github.com/sebhtml/ray/issues/233
Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>
create placeholders for next-generation explorers

Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>