Add a basic sky-scrollable element that scrolls

We still need to polish sky-scrollable, but it basically works.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/875953004

examples/example-scrollable.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.
+-->
+<sky>
+<import src="/sky/framework/sky-element/sky-element.sky" as="SkyElement" />
+<import src="/sky/framework/sky-scrollable.sky" />
+<import src="data/cities.sky" as="cities" />
+
+<sky-element name="example-scrollable">
+<template>
+  <style>
+  sky-scrollable {
+    margin: 20px;
+    height: 400px;
+    border: 2px solid blue;
+  }
+  b {
+    display: inline;
+    font-weight: bold;
+  }
+  </style>
+  <sky-scrollable>
+    <template repeat="{{ cities }}">
+      <template>
+        <div>{{ name }}</div>
+      </template>
+    </template>
+  </sky-scrollable>
+</template>
+<script>
+module.exports = class extends SkyElement {
+  created() {
+    this.cities = cities;
+  }
+}.register();
+</script>
+</sky-element>
+
+<example-scrollable />
+</sky>