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 comment block is now over the 80 character limit, you should make it multi line rather than extending it. Also, multiline comments typically look like this:

```
/*
 * This would be a long
 * explanatory comment.
 */
```

See [here](https://github.com/numpy/numpy/blob/master/doc/C_STYLE_GUIDE.rst.txt) for more code formatting fun.

Both things apply elsewhere in this file.