Using MTDates with GCD throws NSInvalidArgumentException

Executing following code:

```objc
- (void)test_MTDatesBlockInvoke {
    dispatch_queue_t queue = dispatch_queue_create("mtdates-test", NULL);
    dispatch_async(queue, ^{
        NSDate * date = [NSDate dateFromYear:2013 month:1 day:1];

    });
    dispatch_release(queue);
}
```

This snippet will always throw NSInvalidArgumentException with message *setObjectForKey: key cannot be nil*.

This is caused by `[NSOperationQueue currentQueue]` returning nil and then trying to use this return value as a key for thread local dictionary.

I tried to solve this issue with attached pull request. If there is nil returned from currentQueue call address of current thread will be used as a key.