ISO 8601 week extractor

I added a an `isoweek` function which returns the [ISO 8601 week](http://en.wikipedia.org/wiki/Week_number#Week_numbering).  It does what `format(x, '%V')` does on systems that support that conversion specification.  Since I'm on windows, I was out of luck.  The code is adapted from http://en.wikipedia.org/wiki/Talk:ISO_week_date#Algorithms.