BUG: Rename fft C functions to avoid name conflict

Resolves issue #565.

The public fft functions defined in fftpack.c have name conflicts on certain
systems, so the prefix "npy_" was added to these functions.

Added NPY_VISIBILITY_HIDDEN to the public functions in both fftpack.h and
fftpack.c. This required rearranging the includes at the beginning of
fftpack_litemodule.c so that this macro was properly defined. In addition,
Python.h and numpy/ndarraytypes.h were included in the fftpack.h.

One function, rfftb1, was flagged as `static` as it was not exported in the
header.

Build and tests pass using both Python 2.7.8 and Python 3.4.2.