CocoaPods support.

Summary:
This supersedes #309 and #305.

This is still a **work-in-progress.**

**TODO**

 - [x] Create Xcode project that builds an iOS framework
 - [x] Fix failing tests on Xcode
 - [ ] Make `pod lib lint` pass and make sure YogaKit can be included using CocoaPods
 - [ ] Add `pod lib lint` to .travis.yml
 - [x] Migrate to travis osx image with Xcode 8.2

**FOLLOW-UP**
  - [ ] Make YogaKitSample use framework built by new Xcode project.
  - [ ] Make Travis to upload prebuilt versions of the framework. More [here](https://github.com/Carthage/Carthage#use-travis-ci-to-upload-your-tagged-prebuild-frameworks)
  - [ ] Upgrade github/jekyll docs about installation using Cocoapods/Carthage
Closes https://github.com/facebook/yoga/pull/352

Reviewed By: emilsjolander

Differential Revision: D4471950

Pulled By: dshahidehpour

fbshipit-source-id: 8f30c69f9a487b26aa2e5f3b66841334b01b0ab1

YogaKit/Source/YGLayout+Private.h

@@ -0,0 +1,19 @@
+/**
+ * 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 "YGLayout.h"
+#import <yoga/Yoga.h>
+
+@interface YGLayout ()
+
+@property (nonatomic, assign, readonly) YGNodeRef node;
+
+- (instancetype)initWithView:(UIView *)view;
+
+@end