Initial attempt at implementing Display Traits

Initial attempt to get display traits working with ASEnvironment.

To get proper ASDisplayTraits support, you must use an ASViewController. The ASViewController implements UITraitCollection-related methods (`traitCollectionDidChange:`, `willTransitionToTraitCollection:withTransitionCoordinator:`, viewWillTransitionToSize:withTransitionCoordinator`) to update the internal ASDisplayTraits and propagate them to subnodes.

ASTableNode and ASCollectionNode don't actually have their cells as subnodes, so a little bit of trickery is involved (on `setEnvironment:` the table/collection node gets its data controllers completedNodes and propagates the new traits. see `ASDisplayTraitsCollectionTableSetEnvironmentState`). The data controller also passes the current display traits when creating new cells.

ASViewController also supports the ability to return a custom set of display traits. So if you have a modal dialog that should always be told it is in a compact size class, you can set the override block before displaying the VC.

A new example, called Display Traits, has been added. It shows how display traits can be used in a ASViewController with a normal ASDisplayNode as its root, as well as in ASViewControllers hosting table nodes and collection nodes. There is also an example of overriding the default display traits of a VC.

Please provide feedback!

AsyncDisplayKit/Details/ASDataController.h

@@ -17,6 +17,7 @@ NS_ASSUME_NONNULL_BEGIN
 
 @class ASCellNode;
 @class ASDataController;
+@protocol ASEnvironment;
 
 typedef NSUInteger ASDataControllerAnimationOptions;
 
@@ -64,6 +65,11 @@ FOUNDATION_EXPORT NSString * const ASDataControllerRowNodeKind;
  */
 - (void)dataControllerUnlockDataSource;
 
+
+@end
+
+@protocol ASDataControllerEnvironmentDelegate
+- (id<ASEnvironment>)dataControllerEnvironment;
 @end
 
 /**
@@ -122,6 +128,11 @@ FOUNDATION_EXPORT NSString * const ASDataControllerRowNodeKind;
  */
 @property (nonatomic, weak) id<ASDataControllerDelegate> delegate;
 
+/**
+ *
+ */
+@property (nonatomic, weak) id<ASDataControllerEnvironmentDelegate> environmentDelegate;
+
 /**
  *  Designated initializer.
  *