Issue #110: Updates background fetch API

[jleandroperez]

Background Fetch API has been updated. Usage should be as follows:

- (void)application:(UIApplication *)application performFetchWithCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler
{
    [self.simperium backgroundFetchWithCompletion:^(UIBackgroundFetchResult result) {

        completionHandler(result);
    }];
}