Fixed duration to ms conversion

An year was converted to 12 months and a month is 30 days, so a year was 360
days. Fixed that by taking into account full years when computing milliseconds
since unix epoch and fixing duration.asMonth to convert back a year to 12
months.
Refactored * 100 / 100 to toFixed(2)
Merge pull request #684 from ichernev/bug/fix-duration-as-getters

Fix duration "as" getters