Accessibility improvements for Material docs

This change covers a few accessibility fixes for the Material Design docs, including missing alt text, `ng-click` only on the mobile menu icon (a `<div>`) and a way to escape the sidenav via the keyboard. A fuller-featured branch on `material-sidenav` is forthcoming, but at least wiring up the escape key made the docs sidenav easier to use. 

One outstanding issue is in tablet or mobile layout: in the open menu, when you click to view a demo or other component, the docs sidenav stays open on top of the requested content. It should navigate out of the way after successfully loading new content. This would be a good opportunity to work in focus management for the docs, as well: after a user hits the space bar or enter key to view a component, focus should be directed into new content. See screenshot of docs side navigation still open after clicking, tapping, or using keyboard to view component:

<div style="max-width: 100%">
<img alt="Material Design screenshot" src="https://cloud.githubusercontent.com/assets/1045233/4172637/007e2734-354c-11e4-86c6-5396d68900aa.png">
</div>