Added support for colorspace conversion.

Implementation of the fixes discusses in yesterday's meeting.

I think this is quite complete. I've added support for the more generic "from"-"to" convert in ColorSpace, removed any dependency from Topographica, and added documentation (and the __all__specifier).



Won't  % work here, as in 
return numpy.dstack( (L, numpy.hypot(A,B), numpy.arctan2(B,A) % 2pi))

?