Enable resetable and String prefs.

This allows both boolean and string-type preferences. It
also implements a system where prefs that are read from a
configuration file can be reset back to their initial value,
which is useful in a number of cases e.g. when running tests
to ensure that each test starts with the same values for
the prefs.

components/compositing/pipeline.rs

@@ -289,7 +289,7 @@ impl Pipeline {
     pub fn trigger_mozbrowser_event(&self,
                                      subpage_id: SubpageId,
                                      event: MozBrowserEvent) {
-        assert!(prefs::get_pref("dom.mozbrowser.enabled").unwrap_or(false));
+        assert!(prefs::get_pref("dom.mozbrowser.enabled").as_boolean().unwrap_or(false));
 
         let event = ConstellationControlMsg::MozBrowserEvent(self.id,
                                                              subpage_id,