at2x retina image function - fix bug 898013

A new utility script that allows the easy embedding of normal and high-dpi images. Simply call:
```
{{ at2x_img('img/firefox/normal-dpi.png', 'img/firefox/high-dpi.png', 300, 100, **{ 'alt': 'foo'}) }}
```
And an image tag that includes a "data-at2x-src" attribute will be embedded in the page. Javascript is then used to detect high-dpi screens and show the at2x src if supported, otherwise the normal res image is shown.

This also works with platform images:
```
{{ platform_img('img/firefox/normal-dpi.png', **{ 'data-at2x-src': 'img/firefox/high-dpi.png', 'width': '500', 'height': '200'}) }}
```