Tried the first web component

A typical web component will be defined in components/ folder.
A single component contains three files: script.dart template.html and style.css.
The template.html will look like:
```<!DOCTYPE html>
  <body>    
    <element name="gf2-login" constructor="LoginComponent" extends="div">
      <template>
        <link rel="stylesheet" href="components/[component_name]/style.css"> 
        [template content]
      </template>
      <script type="application/dart" src="script.dart"></script>
    </element>
  </body>
</html>```

Seems we need to find a way to store application scope information, like UserInfo.