[Fixes #154] Grids IE7 Fix

This pull request updates the grid units within Pure to fix #154. 

In IE7, the grid can break when you have multiple units than span multiple "rows", without each row being separated by a `.pure-g`. The fiddle over at #154 demonstrates this. This occurs because of IE's sub-pixel rounding. I have fixed this by adding `*width* to the grid to account for this sub-pixel round-up that IE does.