fix(datepicker): mark input tocuhed on calendar date selected

Author: mmalerba

src/lib/datepicker/datepicker-input.ts

@@ -212,6 +212,7 @@ export class MdDatepickerInput<D> implements AfterContentInit, ControlValueAcces
           this._datepicker.selectedChanged.subscribe((selected: D) => {
             this.value = selected;
             this._cvaOnChange(selected);
+            this._onTouched();
             this.dateInput.emit(new MdDatepickerInputEvent(this, this._elementRef.nativeElement));
             this.dateChange.emit(new MdDatepickerInputEvent(this, this._elementRef.nativeElement));
           });