Fix: CSSOM constructors should be available on the DOM.

level2/style.js adds properties to the level2 core instance, so it
shouldn't operate on a cloned version.

lib/jsdom/level2/style.js

@@ -4,9 +4,6 @@ var core = require("./core").dom.level2.core,
     cssom = require("cssom"),
     assert = require('assert');
 
-// modify cloned instance for more info check: https://github.com/tmpvar/jsdom/issues/325
-core = Object.create(core);
-
 // What works now:
 // - Accessing the rules defined in individual stylesheets
 // - Modifications to style content attribute are reflected in style property