iOS GameCenter caching

**Warning: This PR is based on #18 -- so please merge this afterwards.**

With this change the iOS implementation of `avalon::GameCenter` will store scores and achievements in a local file, if the user is not authenticated. As soon as the user is authenticated again all scores / achievements are submitted.

Big thanks to [Minimax Games](http://www.minimaxgames.com/) for sending me a huge patch file per mail. This PR consists mostly out of this patch and a few slight modifications.