ujson remove link dependence on npymath
npymath causing build woes on win32 (#1309 and #1360)

Removed half float support. `npy_isnan` and `npy_isinf` are macros defined in the npymath header so linking isn't necessary. 

Also, build fix for mingw32.

Tested on Python 2.7 on 64-bit OSX, 32-bit Ubuntu and 32-bit WinXP (VS 2008 and mingw32). 
