fix inside list items containing block elements in Firefox

There is an issue with Firefox and list items that include block elements, such as paragraphs and the inside property (according to http://stackoverflow.com/questions/1142314/css-rendering-inconsistency-on-ul-with-firefox-being-the-odd-ball-out).

This fixes the issue.
Merge pull request #5 from ousia/patch-1

fix inside list items containing block elements in Firefox - thanks @ousia