* ASViewController can be used without a node
- If a node isn't provided by developers via -initWithNode:, a default one will be created and used internally.
- This allows developers to use ASViewController like a normal UIViewController and as a base class for all view controllers among which some use a node hierarchy and some don't.
* Update ASDKgram to use a shared base ASViewController
* Minor fixes in ASViewController:
- If its node isn't provided by users, don't replace the view controller's view with the default node's view because it might be loaded from a nib.
- Init a vanilla ASDisplayNode if a node isn't provided.
* Some smaller cleanup
* Remove dummy node for ASViewController if it’s used without a node
* Reimplement IGListKit support in a cleaner way
* Rename and fix some stuff
* Fix supplementaries more
* Update docs
* Update test
* Cleanup minor things
* Tweak it
* Indentation
* Remove irrelevant changes
* Break out cell into its own file
* Fix indentation
* Address feedback
