Refactoring

This does not introduce new features, but organizes project well for future work.
— Networking code moved out of Chain to ChainConnection.
— CoreBitcoin submodule (use $ git submodule update --init).
— Base classes for unit tests in ObjC and Swift.
— Example/Hello Sign uses CocaoPods as users will do.
Do these calls need to be `setToken`, `setBlockchain`, etc?