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/ASCollectionView.mm

@@ -91,7 +91,7 @@ static NSString * const kCellReuseIdentifier = @"_ASCollectionViewCell";
 #pragma mark -
 #pragma mark ASCollectionView.
 
-@interface ASCollectionView () <ASRangeControllerDataSource, ASRangeControllerDelegate, ASDataControllerSource, ASCellNodeLayoutDelegate, ASDelegateProxyInterceptor, ASBatchFetchingScrollView> {
+@interface ASCollectionView () <ASRangeControllerDataSource, ASRangeControllerDelegate, ASDataControllerSource, ASCellNodeLayoutDelegate, ASDelegateProxyInterceptor, ASBatchFetchingScrollView, ASDataControllerEnvironmentDelegate> {
   ASCollectionViewProxy *_proxyDataSource;
   ASCollectionViewProxy *_proxyDelegate;
   
@@ -225,6 +225,7 @@ static NSString * const kCellReuseIdentifier = @"_ASCollectionViewCell";
   _dataController = [[ASCollectionDataController alloc] initWithAsyncDataFetching:NO];
   _dataController.delegate = _rangeController;
   _dataController.dataSource = self;
+  _dataController.environmentDelegate = self;
   
   _batchContext = [[ASBatchContext alloc] init];
   
@@ -917,6 +918,14 @@ static NSString * const kCellReuseIdentifier = @"_ASCollectionViewCell";
   }
 }
 
+- (id<ASEnvironment>)dataControllerEnvironment
+{
+  if (self.collectionNode) {
+    return self.collectionNode;
+  }
+  return self.strongCollectionNode;
+}
+
 #pragma mark - ASCollectionViewDataControllerSource Supplementary view support
 
 - (ASCellNode *)dataController:(ASCollectionDataController *)dataController supplementaryNodeOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath