Node placeholder API

ASDisplayNodes now have an overridable method -placeholderImage that lets you provide a custom UIImage to display while a node is displaying asynchronously. The default implementation of this method returns nil and thus does nothing. A provided example project also demonstrates using the placeholder API.
if we create a node, let it start rendering, then do `node.placeholderEnabled = NO` before it finishes display, will the placeholder layer ever be removed?

perhaps we should teardown the placeholder layer when setting `placeholderEnabled` to `NO`, and set it up (again) if it's set to `YES`.  then this condition could just become

```objective-c
if (_placeholderLayer.superlayer && [self _pendingDisplayNodesHaveFinished]) {
  ...
}
```