Initial code review for google-youtube-video-wall

Hey @ebidel @addyosmani & co:

A rough cut of the `<google-youtube-video-wall>` demo app, as well as the supporting top-level `bower.json`.

A few points to supplement the inline `TODO`s
  - No tests at the moment.
  - I needed to pull in `"paper-elements": "Polymer/paper-elements#master"` in order to pick up a version of `<paper-dialog>` that contained [this patch](https://github.com/Polymer/paper-dialog/commit/11e201a256a2b08699bdecd62824bbd9c0d3cb99). The version tagged 0.3.3 doesn't work properly with the demo.
  - I'm not happy with the performance right now when the `data` that `<core-list>` observes gets concatenated as additional API results are pulled in. There is noticeable flickering as the existing elements seem to be redrawn, and scrolling slows down. I don't know how much of that is `<core-list>`'s fault and how much is my own. I wonder if @sorvell has any recommendations as to what I could be doing incorrectly.
  - I'm eager for feedback about how to make this more Material Design-y.
  - I took a stab at how I thought the `gh-pages` branch should be set up, with the assumption that the bower dependencies for each demo would be referenced via directories under `../bower_components/`. This isn't consistent with the existing model of pulling dependencies directly into the parent directory. I'm happy to change if there are objections.

This branch + the `bower_components` for its dependencies are currently deployed to `gh-pages` and can be accessed at http://googlewebcomponents.github.io/demos/google-youtube-video-wall/demo.html
we're up to 0.3.4