Implements case insensitive font family names.

One part (of 8!) of css font family disambiguation is that font families should
be matched case-insensitively.

This patch implements that. Once it lands, a bug needs to be filed to do lowercasing
properly (as a string, instead of char-by-char -- it's a unicode thing).

r? @gw

components/util/lib.rs

@@ -26,6 +26,7 @@ extern crate sync;
 extern crate task_info;
 extern crate "time" as std_time;
 extern crate string_cache;
+extern crate unicode;
 extern crate url;
 
 #[phase(plugin)]