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)
Added dot net style duration parser
Merge pull request #702 from spring1975/feature/dot-net-duration

Added dot net style duration parser
Merge pull request #684 from ichernev/bug/fix-duration-as-getters

Fix duration "as" getters
Added nodeunit as devDependency

nodeunit is needed as a direct dependency for grunt tasks zone and zones.
Merge pull request #706 from ichernev/bug/nodeunit-dep

Added nodeunit as devDependency
Merge pull request #701 from icambron/durations-as-anything

adding as() and get() to durations
fix as() tests