Add new init function that does not take a key

[mikemee]

Per discussion in #10, it would be nice to have an init function for Amazon that does not require a key, e.g. something like:

public static void init(boolean enableLogging) {
   init(null, enableLogging)
}

public static void init(String key, boolean enableLogging) {
   if(key!=null){        
      billingService.init(key);
   }
   billingService.enableDebugLogging(enableLogging);
}