Fix DateTimeField bugs

When a DateTime property is null or equal to DateTime.MinValue, it will now be correctly removed from the properties instead of being reset to null.

Also the dateFormat for the datepicker and datetimepicker have been changed so it isn't using abbreviated names of the month in it's field value. Because the jquery datepicker's default localization is English, it could conflict with the UserInterfaceCulture (that could be different than English) that is used in the StringToDateTimeConverter, when for example the abbreviated name of the month is used as a value.