Improve python and numpy detection - fixes #12

This should more explicitly help cmake locate the proper python and numpy paths.

I think this should fix issue #12, although it is a bit hard to test because I can't replicate the issue.
I think the main cause of issue #12 is that `PYTHON_EXECUTABLE` isn't set when using homebrew python, and this is used by `FindNumPy.cmake` to set `PYTHON_NUMPY_INCLUDE_PATH`.

However, `PYTHON_NUMPY_INCLUDE_PATH` is still somehow set correctly on my machine, but this clearly isn't the case for everyone.