BUG: Rename fft C functions to avoid name conflict

Resolves issue #565. The fft functions defined in fftpack.c have name
conflicts on certain systems. Because these functions are not exposed, the
prefix "npy_" was added to these functions.

Build and tests pass using both Python 2.7.8 and Python 3.4.2.
this is foreign fftpack code, I would prefer keeping it as similar to the original as possible. I don't think we need to adapt it to numpy coding style as long as we haven't significantly modified it