AjaxObserveField compatible

Now when the CCDatePicker changes the value of the text field it makes a "change event" just as if a user typed in a new date. This allows observers to react such as AjaxObserveField. It mimics the default behavior of AjaxDatePicker and borrows code from that picker.

Signed-off-by: Aaron Rosenzweig <aaron@chatnbike.com>