[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`. [Here's a JSFiddle that demonstrates this](http://jsfiddle.net/C4PQQ/2/). 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.

While I was at it, I also reduced the decimal accuracy from 4 decimals down to 2 decimals. According to [this article](http://nimbupani.com/using-decimal-percentage-values-in-responsive-design.html), the level of accuracy that we had was not necessary. 

### Todo 

- [x] Bump up to 4 decimals.