make sets iterable
Implemented set.add
Implemented set.clear
Implemented set.copy
Implemented set.discard
Implemented set.difference and set.difference_update
Implemented set.intersection and set.intersection_update
Implemented set.isdisjoint
Implemented set.remove
Implemented set.update
Implemented set binary ops.
oops, nasty off-by-one in set_copy
Merge pull request #159 from chipaca/set_feats

Set features. As per #143.