md-card: Add a card footer

[epelc]

Currently you can put your content outside of md-card-content and it will not have padding. But it always put(I assume with a simple ng-transclude) above the content within md-card-content even if you add html below it.

Example

<md-card>
  <h1>My header</h1>
  <md-card-content>
    <p>Lorem ipsum ....</p>
  </md-card-content>
  <span>This should be below the content but it ends up just below the h1</span>
</md-card>

The current hack to have a footer is to set padding-left and padding-right to 0 for md-card-content and then to put your footer within it and add a div with the original padding for your real content. Perhaps an md-card-footer directive could be added. Or elements below the md-card-content could "just work" like how the header works now.