housekeeping. renaming.

--HG--
rename : maths/__init__.py => math/__init__.py
rename : maths/combinatorics.py => math/combinatorics.py
rename : maths/maths.py => math/maths.py
rename : maths/numpy_extras.py => math/numpy_extras.py
rename : maths/weighted_choice.py => math/weighted_choice.py