Remove npymath library from cython extensions

When numpy is built with mingw, the npy_* functions exported from __libnpymath.a__ reference some runtime math functions whose name has a leading underscore, like:
...
000008E5  REL32                      00000000        86  _asinf
000008F5  REL32                      00000000        85  _acosf
00000905  REL32                      00000000        84  _atanf
...
If we try to link this library with visual c by renaming it to npymath.lib it will fail because the corresponding functions in the msvc runtime don't contain that leading underscore:
...
00000281  REL32                      00000000        4F  asinf
00000291  REL32                      00000000        51  acosf
000002A1  REL32                      00000000        53  atanf
...
This means that the library compiled with mingw is not compatible with visual c, and we get these errors:
http://nipy.bic.berkeley.edu/builders/dipy-bdist32-34/builds/76/steps/shell_6/logs/stdio

This PR removes the npymath library from the linking step of the cython extensions, and defines the required subset of math functions in dpy_math.h (by borrowing some code directly from numpy).

With the proposed modification, the conflicting buildbots now pass:
http://nipy.bic.berkeley.edu/builders/dipy-bdist32-33/builds/78
http://nipy.bic.berkeley.edu/builders/dipy-bdist32-34/builds/80