Create Xcode project so it's much easier to run tests.

Summary:
I don't even know how people have been running tests (unless they install BUCK). This creates a workspace to make it much easier to run tests.
Closes https://github.com/facebook/yoga/pull/400

Differential Revision: D4565290

Pulled By: dshahidehpour

fbshipit-source-id: 762168ced81e517126ac05c92bb27ccad0d9d305

YogaKit/Podfile

@@ -0,0 +1,10 @@
+use_frameworks!
+
+workspace 'YogaKit'
+
+target 'YogaKit' do
+  pod 'Yoga', :path => '../Yoga.podspec'
+end
+
+target 'YogaKitTests' do
+end