fix(card): position md-card-footer at bottom of card

Old behavior did not cause the footer to stick to the bottom of the card
if the card had an assigned height taller than its content.

Fixes #3144. Closes #4891.

Author: Michael Chen

src/components/card/card.scss

@@ -28,6 +28,7 @@ md-card {
     }
   }
   md-card-footer {
+    margin-top: auto;
     padding: $card-padding;
   }
 }