HTML center mis-aligns grid

The docs suggest the use of HTML `<center>`, https://github.com/zurb/ink/blob/master/docs/components/grid.php#L187

![localhost4567eventsstandardday-1 html_20131128_161802](https://f.cloud.github.com/assets/154176/1637426/0d124d62-57ea-11e3-92a8-597c9db680b1.png)

This is generated with the following: https://gist.github.com/rjocoleman/7687456
CSS is Ink 1.0.4 only.

If the HTML `center` element is removed the grid behaves as expected.

__Update:__ the placeholder.it of a 580px wide image is probably a bad choice, it's un-related though, litmus tests below.
Converted to a PR.