fix the too-long issue of header line

Hi, talha131, I tried your pelican theme elegant, it's awesome, clean and elegant.

But I find the header line of some pages is too long, e.g. archive.html, tags.html. "span10" makes sense for article.html, cause it needs to cover the side bar column. But it looks weird in archive.html, tags.html, categories.html, when there is no side bar. So I guess perhaps it's a bug?