Add feature detect for nth-child()

Reference: http://reference.sitepoint.com/css/pseudoclass-nthchild

Tested to work in:
Internet Explorer 7 (false), 8 (false), 9 (true)
Chrome 7 (true), 21 (true)
Firefox 3.6 (true), 14 (true)
Safari 5.2 (true), 6 (true)
Opera 9.0 build 3447 (false), 12 (true)

Known issues in:
Safari 3.1, 3.2.2 (false negatives)