support for 12 hour time Locale setting (hourCycle)

[lucianmocan]

fixes #5 When 24-hour time (Locale.current.hourCycle is equal to .zeroToTwentyThree) is toggled ON in Settings, everything works as expected. When 24-hour time is toggled OFF in Settings (Locale.current.hourCycle is equal to .oneToTwelve), certain Date fields (like dstart, dtend) were nil. To fix that issue, the 'formatter' in DateTimeType needs to be set up as "en_US_POSIX" if 24-hour time is OFF.