1. Fix Grid unavailable .offset*

This problem blames to the "ie6-double-margin" problem.
Merge pull request #15 from Witcher42/master

Fix Grid incorrect margin-left of .offset*

If an element has both span* and offset* classes, and it also fits :first-child, the offset* class won't work.

What's more, elements, not :first-child, with offset* and span* have double margin-left too.

As mentioned in #14, this problem blames to the "ie6-double-margin" problem. To solve this problem, just add display: inline; to the element in IE6.