move flushing to a background thread about analytics-ios

[reinpk]

Helpful comments from Jake Heiser:

There are several ways to call -flush on a background thread, mostly depends on how far back you are trying to maintain backwards compatibility. The most used way I've seen is using:

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 
                                         (unsigned long)NULL), ^(void) {
        // Your code
    });

but other ways include using an NSOperationQueue, and -performSelectorInBackground:withObject:. The first and second ways require >= iOS 4.0 but the third goes back further (pretty sure on this one, might want to check).

[reinpk]

fixed by Tony's pull request #6 thanks!