font-feature-settings gecko glue code

FeatureTagValue value property changed to use u32. ToCss for
FeatureTagValue changed to allow conversion from u32 to string. Parse
for the same struct updated to convert from string to u32. Added two
functions to transfer settings to gecko and copy settings.

tests/unit/style/lib.rs

@@ -6,6 +6,7 @@
 #![feature(plugin, test)]
 
 extern crate app_units;
+extern crate byteorder;
 extern crate cssparser;
 extern crate euclid;
 #[macro_use] extern crate html5ever;