Add custom collection view layout example project

examples/CustomCollectionView/Sample/ImageCellNode.m

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+//
+//  ImageCellNode.m
+//  Sample
+//
+//  Created by McCallum, Levi on 11/22/15.
+//  Copyright (c) 2015 Facebook. All rights reserved.
+//
+
+#import "ImageCellNode.h"
+
+@implementation ImageCellNode {
+  ASImageNode *_imageNode;
+}
+
+- (id)initWithImage:(UIImage *)image
+{
+  self = [super init];
+  if (self != nil) {
+    _imageNode = [[ASImageNode alloc] init];
+    _imageNode.image = image;
+    [self addSubnode:_imageNode];
+  }
+  return self;
+}
+
+- (CGSize)calculateSizeThatFits:(CGSize)constrainedSize
+{
+  [_imageNode measure:constrainedSize];
+  return constrainedSize;
+}
+
+- (void)layout
+{
+  _imageNode.frame = CGRectMake(0, 0, _imageNode.calculatedSize.width, _imageNode.calculatedSize.height);
+}
+
+@end