ENH: add support for python ABCs

Beginning with version 2.6, python supports abstract base classes,
which contain a class hierarchy for numbers. This class hierarchy is
very similar to the one of numpy, so it is very easy to register
the numpy type hierarchy with the python type hierarchy.

This patch adds those registrations and also adds unit tests for it.
ENH: add numerator and denominator to integers

numbers.Integral types are supposed to have a numerator and
denominator attribute. This adds those two trivial attributes.
Merge pull request #4547 from tecki/master

add support for python ABCs