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/.swift-version

@@ -0,0 +1 @@
+3.0.2

YogaKit/BUCK

@@ -30,8 +30,8 @@ COMPILER_FLAGS = [
 apple_library(
   name = 'YogaKit',
   compiler_flags = COMPILER_FLAGS,
-  srcs = glob(['*.m']),
-  exported_headers = glob(['*.h']),
+  srcs = glob(['Source/**/*.m']),
+  exported_headers = glob(['Source/**/*.h']),
   frameworks = [
     '$SDKROOT/System/Library/Frameworks/Foundation.framework',
     '$SDKROOT/System/Library/Frameworks/UIKit.framework',

YogaKit/YogaKit.podspec

@@ -0,0 +1,30 @@
+Pod::Spec.new do |spec|
+  spec.name = 'YogaKit'
+  spec.version = '1.0.0'
+  spec.license = { :type => 'BSD-3-Clause', :file => '../LICENSE' }
+  spec.homepage = 'https://facebook.github.io/yoga'
+  spec.authors = {
+    'Emil Sjölander'      => 'sjolander.emil@gmail.com',
+    'Dustin Shahidehpour' => 'dshahidehpour@gmail.com'
+  }
+
+  spec.summary = 'Yoga is a cross-platform layout engine which implements Flexbox.'
+  spec.description = <<-DESC
+  Yoga is a cross-platform layout engine which implements Flexbox.
+  It enables maximum collaboration within your team by implementing an API familiar to
+  many designers and opening it up to developers across different platforms.
+  DESC
+
+  spec.documentation_url = 'https://facebook.github.io/yoga/docs/getting-started/'
+  spec.source = {
+    :git => 'https://github.com/facebook/yoga.git',
+    :tag => 'v2017.01.23.00'
+  }
+  spec.platform = :ios
+
+  spec.dependency 'Yoga', '~> 1.0'
+  spec.source_files = 'Source/*.{h,m}',
+  spec.public_header_files = 'Source/{YGLayout,UIView+Yoga}.h'
+  spec.private_header_files = 'Source/YGLayout+Private.h'
+
+end