intbitset: type checking for operators

* Addition type checking for &, |, etc. operators.
  The type of "self" was not checked.
intbitset: union() and isdisjoint() support

* Adds support for new union() and isdisjoint() set methods.

* Updates intbitset interface to look like set built-in in Python 2.6.

* Supports initialization of an intbitset from a set.
intbitset: no crash when intbitset is on rhs

* When an intbitset is on the right hand side of a foreign type
  operator (e.g. a set), do no longer crash with a segmentation
  fault, rather raises a TypeError.  (closes #1287)

Reported-by: Thorsten Schwander <thorsten.schwander@gmail.com>
Tested-by: Tibor Simko <tibor.simko@cern.ch>