[cssom-values] created

A new "CSSOM Values Module" specification (as ED) is now available at [1].
The intention is to specify all CSSPropertyValue and related APIs in this
separate module rather than in CSSOM [2]. The material defining these
features that was previously in the latter has been moved to this new spec.

[1] http://dvcs.w3.org/hg/csswg/raw-file/tip/cssom-values/Overview.html
[2] http://dvcs.w3.org/hg/csswg/raw-file/tip/cssom/Overview.html