cache gamecenter scores/achievements if not authenticated
cache the highest AND lowest scores; for achievements only the highest
Merge branch 'fix-gamecenter-clear-scores' into ios-gamecenter-caching

* fix-gamecenter-clear-scores:
  add short (but informative *g*) log message

Conflicts:
	avalon/platform/ios/gamecenter/GameCenterIos.m
Merge pull request #19 from michaelcontento/ios-gamecenter-caching

iOS GameCenter caching