Fix for UIDatePickerModeCountDownTimer callback

If presenting an ActionSheetDatePicker with a datePickerMode of
UIDatePickerModeCountDownTimer, currently returns the current date in
the callback.

In this instance, would be much better to return the duration as that
is what the user is picking, not a date
Fix for UIDatePickerModeCountDownTimer callback

If user taps “done” without picking a time, currently returns a
callback of 0 seconds though this is incorrect as the default selection
in the popup is 1 minute (note, user also can’t select 0 minutes).

Fixed so when date picker first instantiated, will set the selected
duration as 60 seconds
Merge pull request #50 from jklp/master

Fix for UIDatePickerModeCountDownTimer callback