Update font-stack with recent findings

Based on conversations in https://github.com/yui/pure/issues/41/, this font stack was updated with more fonts to fully cover all target environments where grids were breaking.

I tested this on:

* IE7,8,9,10
* Latest Chrome, FF, Safari,
* iOS 6.x

@dannyfritz tested this stack on a bunch of environments as well:

* Chromebook Pixel with Chrome OS
* Chromebook Pixel with Ubuntu 13.04 Firefox + Chrome
* Windows 7 Opera + Chrome + Firefox + IE10
* Android 4.2.1 Chrome