IE9 does not string-coerce values, instead throwing an error. We now wrap IE9's
implementation to force string coercion. While it would be simpler to turn on
string-coercion for all browsers inside D3's style operator, this approach
avoids penalizing standards-compliant browsers.
This commit also moves language-compatibility code to a separate directory, and
deletes the obsolete Object.create polyfill, which is no longer needed by D3.
Mike Bostock