Update to reflect changes in Polymer, core-* & paper-& elements

Hey @sethhoward1988. I work with the Polymer team and noticed you've got a great start to the app here. I went through the source and noticed that there are a few improvements that could be made.

This PR will:

* Update `core-drawer-panel` to use the latest API. We've changed the way the `responsiveWidth`, `drawerWidth` and `rightDrawer` attributes work. These changes fix the menu, which was incorrectly rendering with the latest version of Polymer.
* Updated `paper-button` instances to reflect the `label` attribute being deprecated.
* Updated Drive icon references. The icons you were using were moved to different icon-sets in `core-icons` and one was completely removed from the set. I've updated the app to use the correct references to the current versions and added in an element with the one missing icon you were using.
* Added a bower.json and .bowerrc files, allowing you to easily update your checked-in dependencies at any time. This will make the process of API diffing much easier in the future. All checked-in deps have been upated
* We deprecated platform.js, replacing it with webcomponents.js for the polyfills. This PR uses the newer dependency and I've updated any relevant paths.

I've been testing all these changes locally and I haven't been able to find any functionality which broke as a result of these updates. Any questions, feel free to shout. I'd love to help improve the app further.

If we land this, my next PR will probably be shifting over to using the `<google-signin>` element which most other Polymer + Google services apps use over at https://github.com/GoogleWebComponents/google-signin.