Automatically generated CUDA *.pyx based on C/C++ *.pyx

Generating a CUDA version of the *.pyx file is really important in order to avoid having to deal with two files containing more or less the same code.

eg.: with menpo/image/feature/cppimagewindowiterator.pyx
     its CUDA version will be the same except for at most 5 lines among 335

The lookup for *.c, *.cpp, *.cu files, when dealing with nvcc, has also been changed.
These files have to be defined at the top of the *.pyx file.