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.