Make SkyElement more classy.

The syntax for implementing a SkyElement is now:

<sky-element name="element-name">
<template>
  <!-- template here -->
</template>
<script>
module.exports = class extends SkyElement {
  attached() {
     // ...
  }
  // .. methods here ..
}.register();
</script>
</sky-element>

The register() static method on SkyElement subclasses calls
document.registerElement() and returns the generated constructor.
It uses the parent <sky-element>'s name attribute to set the name
of the element.

R=rafaelw@chromium.org

Review URL: https://codereview.chromium.org/788943003