header class for headers that are not <h2>

[valdrinkoshi]

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>

[notwaldorf]

@sorvell says that this is bad news bears (it's really slow, not to mention very deprecated in shadow DOM v1), so I don't think this is a good change. :(

[valdrinkoshi]

So, in light of this, should we deprecate these styles already?

[valdrinkoshi]

Closing this, as it makes the styles slower.