pxd file for the NumPy core math library (npymath)

Here's a Cython wrapper (`.pxd`) for the [NumPy core math library](http://docs.scipy.org/doc/numpy-1.7.0/reference/c-api.coremath.html), npymath. This library makes some C99 and Fortran math functions and constants available in C89 programs, or programs (such as scikit-learn) that need to be portable to such platforms.

I added a unit test. The only problem, AFAIK, that needs addressing before this can be merged is that the tests don't pass with a C++ compiler except with very recent NumPy, because NumPy <=1.7.1 and maybe later did not have a C++ guard in the header file. Is there a Cythonic way to deal with this?