Further support for countdown timer

These changes should fix #72

Added an additional initialiser which allows to set the countdown timer
to an initial countdown interval.

Changed picker view configuration regarding the existence of countdown
timer.
Merge branch 'master' of https://github.com/mgmart/ActionSheetPicker-3.0 into mgmart-master
prettify code, move duration to public interface
add swift examples for all type of DatePickers
update property name
update example
- General fixes for #74 & #50
- Remove unused init function.
- Add CountDownTimer support to blocks.
- Remove redudant countDownDuration init
Merge pull request #76 from skywinder/countDownDatePickerUpdate

- General fixes for #74 & #50