col-offset-* makes columns smaller instead of moving them

prior to this patch col-offset-* was applying a width instead of a
margin-left to divs making columns smaller instead of pushing them to
the right. Verified correct css from twitter/bootstrap repo:
twitter/bootstrap bootstrap.css on 3.0.0-wip branch starting at line 895
```
.col-offset-1 {
    margin-left: 8.333333333333332%;
  }
```