add isoweek extractor to extract the ISO8601 week.
add isoweek accessor test for a tricky date
Couple of changes based on hadley's suggestions on my pull request
ensure integer return value and handle dates in isoweek

Final division changed to integer division in isoweek.  In addition,
input coerced to POSIXct using ISOdate to ensure classes match for
substraction of nearest thursday (nth) and january 1st (jan1).
Previously, date input resulted in Date - POSIXct.
Add isoweek vector tests to test-accessors.R
Merge branch 'master' of git://github.com/jasonelaw/lubridate into jasonelaw-master