-moz-osx-font-smoothing for Firefox font smoothing support.

-moz-osx-font-smoothing is the analog of -webkit-font-smoothing in
Firefox in OS X (where it's most needed).  This change adds support for
-moz-osx-font-smoothing where -webkit-font-smoothing is currently used.
Merge pull request #201 from philipn/fix_firefox_font_smoothing

-moz-osx-font-smoothing for Firefox font smoothing support.