Clean up ASDisplayNode internal flags.

* Reorganise and rename `_flags` for clarity and consistency.
* Remove ambiguity between `implementsDisplay` and `hasClassDisplay`.
* Delete useless `hasWillDisplayAsyncLayer` check -- make it a simple
  subclass override point, as with `didDisplayAsyncLayer:`.
* Minor comment cleanup.
ASDisplayNode.inWindow -> .inHierarchy.

This is a remnant from before `willAppear` and `didDisappear` were
renamed to better reflect their hierarchy-change status (being added to
the hierarchy != being visible).  May be useful for turians.
.preventOrCancelDisplay -> .displaySuspended.

Rename the ASDisplayNode property to match its _ASDisplayLayer
counterpart -- `displaySuspended` is more succinct and is a more
plausible name for a Cocoa BOOL property.
Merge pull request #128 from facebook/cleanup

Miscellaneous cleanup.