Support for FFTs

This wraps the FFTW library to do FFTs in libdynd.
Nothing wrong, just I think we had different ideas about things: I had nd::empty pegged as the convenient API for numpy-like construction, and nd::dtyped_empty as the exact same API function but that  takes the raw shape array, so that the ambiguity is avoided by there being different names. That's not satisfactory, though, but neither is requiring users of the library to have to know that nd::empty(0, NULL, dtype) would create a 2D 0x0 array of the given dtype instead of a scalar.