Fix issue 15: "Week calculation is wrong ... includes decimals"

According to the updated article,
http://techblog.procurios.nl/k/n618/news/view/33796/14863/calculate-iso-8601-week-and-year-in-javascript.html
either ceil or floor is needed. Choosing to use floor to keep the +1.
Merge pull request #19 from felixge/issue15

Fix issue 15: "Week calculation is wrong ... includes decimals"