add datepickerStyleClass to calendar datepicker

[dhniels]

dropdown has a panelStyleClass attribute to target the .ui-dropdown-panel element with your own custom class, however, the calendar does not have the similar ability to target the datepicker that appears when clicking the input. it is important to add this feature to allow users to assign a custom class to the datepicker. a property like datepickerStyleClass would allow this to happen.