Added support for CountDownPicker.

There is still a bug in UIDatePickerMode.CountDownTimer. The first valueChanged action is not send to the target.
I found a workaround[1] for this bug. But it does not work in this context.

[1] http://stackoverflow.com/questions/20181980/uidatepicker-bug-uicontroleventvaluechanged-after-hitting-minimum-internal