utils::url::open shouldn't be allowed to crash the whole app
remove dangerous code in GameCenter clearAllScores

it's not safe to simply submit 0 as new score. just think of scores sorted from
low to high. the user would jump straight to the top - no good!
cache gamecenter scores/achievements if not authenticated
cache the highest AND lowest scores; for achievements only the highest
return bool from the obj-c backend to c++
check delegate pointers in iOS backend before using them
Merge pull request #16 from michaelcontento/catch-url-open-exceptions

utils::url::open shouldn't be allowed to crash the whole app [try-catch block added]
Merge pull request #21 from michaelcontento/check-ios-payment-delegate-pointer

Check payment::ManagerDelegate pointers in iOS backend before usage
add short (but informative *g*) log message
Merge pull request #18 from michaelcontento/fix-gamecenter-clear-scores

remove dangerous code in GameCenter clearAllScores
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
Merge pull request #20 from michaelcontento/gamecenter-return-bool-on-show-methods

GameCenter return bool on show methods