Revert removal of paper-shadow.css

https://github.com/Polymer/paper-shadow/commit/74473f7c35bb9a88d916cc297955e646ddb394b7 broke the class examples, I opened https://github.com/Polymer/paper-shadow/issues/16 to highlight why this actually happened, as I think its probably unintentional (it seems wrong to me to be including css from a custom element directly).