[Examples] CustomCollectionView in Swift3 (#2591)

* new Swift example

* removed asyncDataSource and asyncDelegate. Also using collectionNode methods instead of collectionView

* Fixing the folder structure

* Updated with @george-gw feedback. Thanks George.

examples/CustomCollectionView-Swift/Sample/ImageCellNode.swift

@@ -0,0 +1,51 @@
+//
+//  ImageCellNode.swift
+//  Sample
+//
+//  Created by Rajeev Gupta on 11/9/16.
+//
+//  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.
+//
+//  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+//  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+//  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+//  FACEBOOK BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+//  ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+//  CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+//
+
+import UIKit
+import AsyncDisplayKit
+
+class ImageCellNode: ASCellNode {
+  let imageNode = ASImageNode()
+  required init(with image : UIImage) {
+    super.init()
+    imageNode.image = image
+    self.addSubnode(self.imageNode)
+  }
+
+  override func layoutSpecThatFits(_ constrainedSize: ASSizeRange) -> ASLayoutSpec {
+    let imageSize = imageNode.image?.size
+    print("imageNode= \(imageNode.bounds), image=\(imageSize)")
+    
+    var imageRatio: CGFloat = 0.5
+    if imageNode.image != nil {
+      imageRatio = (imageNode.image?.size.height)! / (imageNode.image?.size.width)!
+    }
+    
+    let imagePlace = ASRatioLayoutSpec(ratio: imageRatio, child: imageNode)
+    
+    let stackLayout = ASStackLayoutSpec.horizontal()
+    stackLayout.justifyContent = .start
+    stackLayout.alignItems = .start
+    stackLayout.style.flexShrink = 1.0
+    stackLayout.children = [imagePlace]
+    
+    return  ASInsetLayoutSpec(insets: UIEdgeInsets.zero, child: stackLayout)
+  }
+  
+}