Update polymer.dart homepage

This is a first crack at updating the polymer page to reflect core & paper, and to remove the big gray hero box. (I need to update the Dart homepage, as well, but that'll be later.)

Staged at:

https://demos-dot-dart-lang.appspot.com/polymer-dart/

R=sigmund@google.com

Review URL: https://chromiumcodereview.appspot.com/393203002