Added inverse DFT using FFTW, and related test.

DFTs can be defined in a few different ways.  I've gone with
the convention of numpy: the forward DFT is un-normalized and
the inverse has a (1/n) prefactor.  See:
http://docs.scipy.org/doc/numpy/reference/routines.fft.html

Test checks that the forward and backward transforms are inverses
of one another.