PR: Implement "__pow__" magic method support for "colour.SpectralPowerDistribution" and "colour.TriSpectralPowerDistribution" classes.

I implemented the __pow__ magic for "colour.SpectralPowerDistribution" and "colour.TriSpectralPowerDistribution" classes as per issue #110.

I didn't change any ipython notebooks or API documentation, let me know if I should do that as well.

lukec