fix(datepicker): week count issues

This fixes a few issues with week counting

(1) Week counts in January are counted as last years' weeks when the first Thursday is from last year (e.g., 53, 54, 55, ...).  See January 2010, for example.

(2) Week counts in December continue counting the last week(s), which may be part of next year, as weeks from the current year.  See December 2014, for example.

(3) Week counting changes depending on the start day.  The week containing June 7th 2014 is week 22 with the default start day (Sunday), but week 23 with start day Monday.  See examples on http://angular-ui.github.io/bootstrap/ (popup has start day Monday)

    The code has been changed so that the Thursday of the week always determines, which week count the week gets assigned, regardless of the start day of the week.