Adding missing .offset-xs0-important class

[davidmarquis]

The class offset-xs0-important is missing, resulting in bad margins for small devices (xs-sized) when other offsets are used for larger screen sizes.

My case is I want to have a section that uses a centered portion of the screen for larger screens, but the whole screen (12 columns) when in xs form.

I would expect these classes to give that effect (each screen size defines a different offset):

<iron-grid>
      <div class="offset-xs0 xs12 offset-s2 s8 offset-m3 m6 offset-l4 l4">
      ...
     </div>
</iron-grid>

Because there's no offset-xs0-important class to force margins to 0 for xs-sized viewports in iron-grid, then the margins of the last offset-type class applied on the element (in this case offset-l4) prevail.

This plunkr demonstrates the issue: http://embed.plnkr.co/rweaGISKxSqtakB775ib/

[maxiplay]

I valid. I forgot it.

[ozwaldorf]

Merged #24