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/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',