[LayoutSpecExamples] Added swift counterpart. (#2576)

* [LayoutSpecExamples] Added swift counterpart.

* Fixed copyright text.

examples/LayoutSpecExamples-Swift/Sample/LayoutExampleViewController.swift

@@ -0,0 +1,46 @@
+//
+//  LayoutExampleViewController.swift
+//  Sample
+//
+//  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 AsyncDisplayKit
+
+class LayoutExampleViewController: ASViewController<ASDisplayNode> {
+
+  let customNode: LayoutExampleNode
+
+  init(layoutExampleType: LayoutExampleNode.Type) {
+    customNode = layoutExampleType.init()
+
+    super.init(node: ASDisplayNode())
+    self.title = "Layout Example"
+
+    self.node.addSubnode(customNode)
+    let needsOnlyYCentering = (layoutExampleType.isEqual(HeaderWithRightAndLeftItems.self) || layoutExampleType.isEqual(FlexibleSeparatorSurroundingContent.self))
+
+    self.node.backgroundColor = needsOnlyYCentering ? .lightGray : .white
+
+    self.node.layoutSpecBlock = { [weak self] node, constrainedSize in
+      guard let customNode = self?.customNode else { return ASLayoutSpec() }
+      return ASCenterLayoutSpec(centeringOptions: needsOnlyYCentering ? .Y : .XY,
+                                sizingOptions: .minimumXY,
+                                child: customNode)
+    }
+  }
+  
+  required init?(coder aDecoder: NSCoder) {
+    fatalError("init(coder:) has not been implemented")
+  }
+}