add xcproject and tests target

[aboedo]

Does a few changes:

• Adds a new xcode project, which gives us the ability to compile the files here to make sure that they work correctly.
• The xcode project compiles a framework, which should be the preferred way to use the hybrid common stuff in the future, but updating isn't strictly necessary yet.
• [BREAKING CHANGE] Moves the source files from ios/ to ios/PurchasesHybridCommon/PurchasesHybridCommon/. References will have to be updated.
• Adds test target, configured with Quick tests.
• Adds a sample test
• Sets up purchases-ios as a git submodule of purchases-hybrid-common.
  This doesn't make a difference for SDKs that just pull the files from common, they should still work as usual. However, if you use the PurchasesHybridCommon.framework, you need to make sure that the Purchases.framework can be located by hybrid common in framework search paths. We'll automate this a bit more in the future.

For development purposes, you'll need to git submodule init to make sure that purchases-ios gets checked out so that you can compile the xcode project.

[vegaro]

oh this is a good idea

[vegaro]

fancy