fix(DlDateTimePickerComponent): event reference should have been $event

This  resulted in a referenceError in DlDateTimePickerComponent._handleKeyDown method.
Merge pull request #426 from steven-hadfield/master

src/lib/dl-date-time-picker/dl-date-time-picker.component.ts

@@ -415,7 +415,7 @@ export class DlDateTimePickerComponent<D> implements OnChanges, OnInit, ControlV
 
       this.focusActiveCell();
       // Prevent unexpected default actions such as form submission.
-      event.preventDefault();
+      $event.preventDefault();
     }
   }