Adding range-restricted enumerations, and counting to this pull
request.  Some basic tests, but more are needed.

new file:   enumerative.py
    Now contains all implementations
modified:   iterables.py
    moved multiset_partitions_taocp to enumerative
new file:   tests/test_enumerative.py
    at least a start on tests.
modified:   tests/test_iterables.py
    moving some tests into test_enumerative