Move sky-*.sky into framework/elements

sky/examples/stocks/stock.sky

 <!--
 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 -->
-<import src="/sky/framework/material-element.sky" />
-<import src="/sky/framework/sky-element.sky" />
+<import src="/sky/framework/elements/material-element.sky" />
+<import src="/sky/framework/elements/sky-element.sky" />
 <import src="stock-arrow.sky" />
 
 <sky-element>
 <template>
   <style>
   :host {
     // TODO(eseidel): Why does setting height here make this too big?
     // height: 48px;
     max-height: 48px;
     display: flex;
@@ skipping 45 matching lines @@
     change.textContent = changeString;
 
     StockArrow arrow = shadowRoot.getElementById('arrow');
     arrow.change = model.percentChange;
   }
 }
 
 _init(script) => register(script, Stock);
 </script>
 </sky-element>