Initial progress of complex color contrast function
Improvements to contrastCheck function

Use Compass helper pow() to raise the expressions that calculate the
R, G and B components by the power of 2.4 (as per W3C equation for
relative luminance).
Use lightness() function to brigten a foreground color in the same
color range to increase the contrast.

TODO: if brightening the color *still* doesn't achieve enough contrast
then darken the color until it does.
Tidy up contrast function. Need to work on $percent calculation
Fix color function so it returns ratios correctly
Abandon idea of returning color - just warn if not enough contrast
Add first draft of Compass mixins and helpers
First draft of Sprites section
First draft of Wrapping up and Resources sections