Callback queue

Great library!

One concern is the requirement to always call back on the main queue. This makes a very sensible default, but I've had one or two situations where its been necessary to call back on some other queue. 

Example of BBHTTP calling back on another queue: 

``` objc
[request setCallbackQueue:dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)];
```

. . what do you think? 
