Additional custom action and negative event tracking

[DZabavchik]

Two features and one problem fix:

• Count negative events (crashes, connectivity issues, etc.). Do not attempt to ask for review if number of negative events exceeds threshold

void uncaughtExceptionHandler(NSException *exception) {
    [Appirater userExperiencedNegativeEvent];
}

• Add custom button to alert view and handle user selecting new custom action in delegate. Example: Allow user to report a problem instead of leaving negative review

It also fixed the problem setting the delegate. _delegate was set in class method.