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.
hmm.  instead of providing a `size` argument and passing in `_size`, why not tell people to use `self.calculatedSize` and any other layout information they've cached in `-measure:`?  then the signature would simply become
```objective-c
- (UIImage *)placeholderImage;
```