Add needed subset of numpy functions and remove npymath library from linking
BF: add npymath info but remove the library from linking
BF: NPY_HAVE_DECL_SIGNBIT is true but _signgit is undefined in vc runtime
BF: check definition of signbit directly
RF: remove check_npymath
BF: explicitly request manifest creation to msvc
BF: /MANIFEST flag no longer needed
PEP8: remove extra blank lines
RF: use get_include instead of get_info
Merge pull request #527 from omarocegueda/remove_npymath

MRG: Remove npymath library from cython extensions

npymath lib does not work for MSVC and numpy compiled with mingw.  Rather than work round this, copy the functions we need into `dpy_math.h` and use those.