Support for high-resolution displays

## About this commit

This commit adds support for high-resolution displays so the HTML5 `canvas` is properly scaled and displayed in the proper resolution.

## Before

![before](https://cloud.githubusercontent.com/assets/1239874/4346786/996f0aa6-4115-11e4-9ac1-d9e22d70e4bc.png)

## After

![after](https://cloud.githubusercontent.com/assets/1239874/4346785/99660ea6-4115-11e4-8ee8-b00414249501.png)

## Test

Tested on:
* Current Safari, Chrome, and Firefox on OS&nbsp;X with a Retina display
* Mobile Safari on iOS

Please test on other devices if you want. :relaxed: