Issue #55 was fixed, give credit
Re-implement IPSet.intersection() without using IntSet
Re-implement difference() and symmetric_difference() without IntSet

This removes helper function partition_ips() and adds helper functions
_subtract() and _iter_merged_ranges().
Remove intset.py

Also remove license information, unit tests and (now unused) imports.
More tests for IPSet operations

They especially target the methods that were re-implemented to not use
IntSet.