Preventing grids distributing verticle space

When there's a grid container of a defined height excess vertical space gets distributed across the rows rather than allowing each row to take up its natural height [see this fiddle](http://jsfiddle.net/ambekpdm/). As grids are used for horizontal positioning it's pretty counter intuitive to come across it affecting vertical distribution. This can be fixed by adding `align-content: flex-start` to the element with the `pure-g` class.

Not sure if this is something best fixed by adding this style in to Pure itself or something to highlight on the site along with the box-sizing notes. But considering it causes significantly different behaviour in browsers without flexbox I'd consider this to be the best place.