Merge in downstream changes.

Introduce `ASTableView`, a UITableView subclass that uses `ASCellNode`
instead of UITableViewCell.  Add working range support via
`ASRangeController`, which observes the visible range, maintains a
working range, and handles most ASDK machinery.  ASRangeController is
loosely-enough coupled that it should be easily adapted to
UICollectionView if that's desired in the future.

Notable considerations in the ASRangeController architecture:

* There's no sense rewriting UITableView -- the real win comes from
  using nodes instead of UITableViewCells (easily parallelisable
  computation, large number of cells vs. few table views, etc.).  So,
  use a UITableView with empty cells, using UITableViewCell's
  contentView as a host for arbitrary node hierarchies.

* Instead of lazy-loading cells the instant they're needed by
  UITableView, load them in advance.  Preload a substantial number of
  nodes in the direction of scroll, as well as a small buffer in the
  other direction.

* Maintain compatibility with UITableView's API, with one primary change
  -- consumer code yields configured ASCellNodes, not UITableViewCells.

* Don't use -tableView:heightForRowAtIndexPath:.  Nodes already compute
  their preferred sizes and cache results for use at layout-time, so
  ASTableView uses their calculatedSizes directly.

* Corollary:  ASTableView is only aware of nodes that have been sized.
  This means that, if a cell appears onscreen, it has layout data and
  can display a "realistic placeholder", e.g. by making its subnodes'
  background colour grey.

Other improvements:

* Remove dead references and update headers (fixes #7, #20).

* Rename `-[ASDisplayNode sizeToFit:]` to `-measure:` and fix
  `constrainedSizeForCalulatedSize` typo (fixes #15).

* Rename `-willAppear` and `-didDisappear` to `-willEnterHierarchy` and
  `-didExitHierarchy`.  Remove `-willDisappear` -- it was redundant, and
  there was no counterpart `-didAppear`.

* Rename `viewLoaded` to `nodeLoaded`.

AsyncDisplayKit/ASCellNode.m

@@ -0,0 +1,109 @@
+/* Copyright (c) 2014-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+#import "ASCellNode.h"
+
+#import "ASDisplayNode+Subclasses.h"
+#import "ASRangeControllerInternal.h"
+#import "ASTextNode.h"
+
+
+#pragma mark -
+#pragma mark ASCellNode
+
+@interface ASCellNode () {
+  // used by ASRangeController machinery
+  NSIndexPath *_asyncdisplaykit_indexPath;
+}
+
+@end
+
+
+@implementation ASCellNode
+
+- (instancetype)init
+{
+  if (!(self = [super init]))
+    return nil;
+
+  return self;
+}
+
+// TODO consider making this property an associated object in ASRangeController.mm
+- (NSIndexPath *)asyncdisplaykit_indexPath
+{
+  return _asyncdisplaykit_indexPath;
+}
+
+- (void)setAsyncdisplaykit_indexPath:(NSIndexPath *)asyncdisplaykit_indexPath
+{
+  if (_asyncdisplaykit_indexPath == asyncdisplaykit_indexPath)
+    return;
+
+  _asyncdisplaykit_indexPath = [asyncdisplaykit_indexPath copy];
+}
+
+@end
+
+
+#pragma mark -
+#pragma mark ASTextCellNode
+
+@interface ASTextCellNode () {
+  NSString *_text;
+  ASTextNode *_textNode;
+}
+
+@end
+
+
+@implementation ASTextCellNode
+
+static const CGFloat kHorizontalPadding = 15.0f;
+static const CGFloat kVerticalPadding = 11.0f;
+static const CGFloat kFontSize = 18.0f;
+
+- (instancetype)init
+{
+  if (!(self = [super init]))
+    return nil;
+
+  _textNode = [[ASTextNode alloc] init];
+  [self addSubnode:_textNode];
+
+  return self;
+}
+
+- (CGSize)calculateSizeThatFits:(CGSize)constrainedSize
+{
+  CGSize availableSize = CGSizeMake(constrainedSize.width - 2 * kHorizontalPadding,
+                                    constrainedSize.height - 2 * kVerticalPadding);
+  CGSize textNodeSize = [_textNode measure:availableSize];
+
+  return CGSizeMake(ceilf(2 * kHorizontalPadding + textNodeSize.width),
+                    ceilf(2 * kVerticalPadding + textNodeSize.height));
+}
+
+- (void)layout
+{
+  _textNode.frame = CGRectInset(self.bounds, kHorizontalPadding, kVerticalPadding);
+}
+
+- (void)setText:(NSString *)text
+{
+  if (_text == text)
+    return;
+
+  _text = [text copy];
+  _textNode.attributedString = [[NSAttributedString alloc] initWithString:_text
+                                                               attributes:@{NSFontAttributeName: [UIFont systemFontOfSize:kFontSize]}];
+
+  [self invalidateCalculatedSize];
+}
+
+@end