Adds Bower support

This pull request:

* Adds a bower.json file
* Checks in bower_components with platform.js and polymer.html as agreed
* Changes paths in index.html to point to the Bower Platform.js
* Adds path to my-element pointing to polymer.html

This PR does not:

* Add Bower install instructions to the readme
* Change the CDN paths recommended for usage in the readme

I think we should decide on these in a separate PR.