Fixes #35, fixes #21 by avoiding the setting of margin, padding for the element with header class.
Also, made styles specific enough not to be overridden by something as simple as

<style is="custom-style">
  h2 {
    margin: 0;
    padding: 0;
  };
</style

Use case for #35: paper-toolbar has a background color and buttons on the left/right, and it handles padding itself, so it should use this class when used in a paper-dialog like this

<paper-dialog>
  <paper-toolbar class="header">
    <div class="title">Header</div>
  </paper-toolbar>
  <div>Content</div>
</paper-dialog>