Reorganizations, inverses, and tests

1. Add code for the XYZ -> sRGB conversion
2. Add unit tests to srgb
3. Reorganize cam02ucs introducing (placeholder) inverse transformations
4. Add "J/K a' b'" which is the implicit uniform space in Luo et al. (2006). (Continuing the long tradition of semantic transparency in color jargon!)

Todo: 
1. No real unit tests for the UCS stuff, except to test that J/K a' b' gives the same results as the old similarity function. 
2. Need to write the function J'a'b' -> JMh. 
3. Need to interface to the function JMh -> XYZ from Billy Biggs' C code. (Can you do this? I have never used Cython for anything more than stuff like "cdef int i;" so I'm not sure what to do)