Move NIR check digit calculation to function
Move NIR check digit calculation to function

This also fixes a number of formatting issues, improves the module
description and adds tests for the 2A and 2B departments.

arthurdejong/python-stdnum,holvi/python-stdnum,arthurdejong/python-stdnum,holvi/python-stdnum,holvi/python-stdnum,arthurdejong/python-stdnum