fix to addDay - use setDate instead of setTime, crossing DST problem.

Using setTime in addDay() and crossing over DST change date gives:
before setTime: Sun Oct 27 2013 00:00:00 GMT+0300 (Jerusalem Daylight Time)
after setTime: Sun Oct 27 2013 23:00:00 GMT+0200 (Jerusalem Standard Time)

Actually staying on same day and going to infinite loop.
Merge pull request #23 from Izikd/master

fix to addDay - use setDate instead of setTime, crossing DST problem.