Move playground to its own repo

[hwillson]

We want to move the Playground with examples of how to set things up for Apollo-iOS to its own repo for a couple of reasons:

• Discoverability: Most people have no idea it exists. Having its own repo will help with visibility
• Decoupling: This will help decouple changes to the repo vs. changes to the playground. This will allow us to update the playground when a new version is released more easily, and not have a mix of released and unreleased changes in the playground.
• Carthage: Carthage automatically builds every shared scheme in a given xcodeproj in the root of a repo. There is not a way within Carthage to exclude or include specific schemes. This is a real pain for the playground scheme since it builds all sub-libraries, and vastly bloats Carthage build time for our library.