fix(datepicker): mark input touched when date is selected on claendar

Author: mmalerba

src/lib/datepicker/datepicker-input.ts

@@ -211,6 +211,7 @@ export class MdDatepickerInput<D> implements AfterContentInit, ControlValueAcces
       this._datepickerSubscription =
           this._datepicker.selectedChanged.subscribe((selected: D) => {
             this.value = selected;
+            this._onTouched();
             this._cvaOnChange(selected);
             this.dateInput.emit(new MdDatepickerInputEvent(this, this._elementRef.nativeElement));
             this.dateChange.emit(new MdDatepickerInputEvent(this, this._elementRef.nativeElement));