[pymorse] print message if no concurrent.futures

Following the message from Brent Komer <brent.komer@gmail.com> Fri, Oct 11, 2013 at 6:09 PM on morse-users@laas.fr.

Shall we put `concurrent.futures` in a `requirements.txt` for pypi ?
something like: https://github.com/ross/requests-futures/blob/master/requirements-python-2.7.txt