gHorizontalPercentageRegExp is too greedy causing incorrect Margin flips

The gHorizontalPercentageRegExp regexp is too greedy causing it to modify the next rule which it finds that contains a % symbol.

This was brought up in http://core.trac.wordpress.org/ticket/26181

This pull request includes a test case, and a fix that is unlikely to be optimal, but fixes this specific case for me good enough to show the problem.
The Python CSSJanus does not appear to be affected by this in testing.