Add "grey" variants for "gray" colors

The non-American spelling of grey is widely supported, and explicitly
in the SVG color spec http://www.w3.org/TR/css3-color/#svg-color
Fixes gh-86
Fixes stubbornella/csslint#407
Merge pull request #87 from nschonni/add-grey-colour-aliases

Add "grey" variants for "gray" colors