Comparison with other types

* Comparing an intbitset with anything that is not an intbitset
  should behave as a set would. E.g.
  intbitset([1,2,3]) == set([1,2,3]) is False.
  (closes #18)

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Reported-by: Dimitrios Semitsoglou-Tsiapos <dsemitso@cern.ch>