Scale Canvas to "fullscreen" with aspect ratio

Hi,

I've added an resize function and event listener to scale the canvas with correct aspect ration. I've set the canvas size to 320x480 (Portrait mode) initially. Maybe it's a small step towards [Issue #8](https://github.com/mozilla/mortar-game-stub/issues/8) 

There are some more changes:
- Hint is now been shown as a overlay which could be hidden by click.
- Install button is positioned at the bottom and floats over the game canvas. 

The code is based on: https://hacks.mozilla.org/2013/05/optimizing-your-javascript-game-for-firefox-os/