Add a basic sky-element custom element

sky/examples/example-element/index.sky

Index: sky/examples/example-element/index.sky
diff --git a/sky/examples/color/color-picker-demo.sky b/sky/examples/example-element/index.sky
similarity index 53%
copy from sky/examples/color/color-picker-demo.sky
copy to sky/examples/example-element/index.sky
index e8d88e58b3dd0ae3870e145ab598e6682986579c..f895fbefd4f70e8836f6169492ad74cbadef210c 100644
--- a/sky/examples/color/color-picker-demo.sky
+++ b/sky/examples/example-element/index.sky
@@ -1,10 +1,16 @@
-#!mojo mojo:sky_viewer
 <!--
 // 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.
 -->
 <sky>
-<import src="color-picker.sky" />
-<color-picker />
+  <import src="example-element.sky" />
+  <style>
+    example {
+      border: 1px solid blue;
+      width: 100px;
+      height: 100px;
+    }
+  </style>
+  <example />
 </sky>