No segmentation fault with "None <op> intbitset"

* Raises TypeError rather than terminating current process with a
  segmentation fault when None is used on the left side of an
  operation with an intbitset, e.g. in:
     None - intbitset()
     None - intbitset([1,2,3])
     None & intbitset()
     None | intbitset()
  (closes #13)

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Reported-by: Federico Poli <federpoli@gmail.com>