Commented out overflow checking in bits, and added a bit_as_pair operator.  Added code to the basic atom comparator to correctly compare bit strings.  Changed the "value" of the `BitString` instance to be a tuple, as with floats.
Added comment on the bit string stuff to the CHANGES file.
Changed `bit_as_pair` to `bit_as_signed_pair` and `bit_as_unsigned_pair`.  Added an `is_length` operator.  Bit strings remember if they were declared negative, and keep that as a display preference.