ENH: Use Slicer/ParameterSerializer and update to HEAD.

The upstream repository location is now github.com/Slicer/ParameterSerializer.

Also update the ExternalProject version to latest HEAD which contains
the latest fixes for Slicer.
Merge pull request #292 from thewtex/ParameterSerializerSlicer

ENH: Use Slicer/ParameterSerializer and update to HEAD.
ENH: Fixed FFT (pads and handled non-zero indexes)
BUG: Fixed FFTGaussianDeriv - must perform FFT on gauss derivs
Merge pull request #293 from aylward/FFT

Fft
BUG: FFT output type defaults to depending on input type. Needs conversion.
Merge pull request #294 from aylward/FFT

BUG: FFT output type defaults to depending on input type. Needs conversi...