update support for iterables

* Amends .update() and corresponding methods to accept also
  non-intbitset objects, such as lists or sets of integers respecting
  the set interface.
  (closes #17)

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Reported-by: Brent Pedersen <bpederse@gmail.com>