* [ASVideoNode] Change superclass to ASNetworkImageNode so that ASVideoNode can be its own placeholder
- remove _placeholderImageNode property of ASVideoNode (use self.image now instead)
- move layoutSpecThatFits: code to calculateSizeThatFits: & layout: methods as ASImageNode uses calculateSizeThatFits:
* [ASVideoNode] Tweaks to the definition of the delegate protocols to integrate with ASNetworkImageNode (superclass)