Update histogramViewer.py

Adding functionality to convert RGB color space to XYZ, then to sRGB by multiplication.
We then will plan to draw the sRGB histogram lines. 

Straight and to the point source: http://rsb.info.nih.gov/ij/plugins/download/Color_Space_Converter.java

The theoretical and rather, mathy source: 
http://www.ryanjuckett.com/programming/rgb-color-space-conversion/
Merge pull request #3 from jrdk/master

Update histogramViewer.py