Gamecenter return bool on show methods

**Warning: This PR is based on #19 -- so please merge this afterwards**

This change returns a `bool` in `GameCenter::showScores` and `GameCenter::showAchievements`, to indicate if the GameCenter view is now visible.

In the old behaviour we just triggered `GameCenter::login`, which could have opened the login view again. But -- and this the important point -- nothing would happen if the user has disabled and/or ignored the GameCenter. So it feels like a dead button - not cool.

With this change the developer can simply issue a short "please login/enable the GameCenter"-alert, remove the GameCenter buttons or whatsoever.