Specify the thread where callback handler is executed

[kumabook]

Hi,

I have used APIKit and I want to say thank you.

I'm trying to use this library for command line application with Commandant and ReactiveCocoa.
I tried to use SignalProducer#single() in order to get result synchronously, but it doesn't get result.
Currently, callback handlers always are executed on main thread.
For command line application, it is good to specify the thread where callback handler are executed.
I tried to modify to specify the thread, it works for me.
Please use it as a reference.

Thank you.

[ishkawa]

Looks nice 👍

It would be good to extend this for NSOperationQueue and private queue of Session:

public enum CallbackQueue {
    case Main
    case SessionQueue
    case OperationQueue(NSOperationQueue)
    case DispatchQueue(dispatch_queue_t)
}

[ishkawa]

I'll work on another PR soon. Thank you for the nice suggestion ✨

[ishkawa]

Closing this in favor of #157.

@kumabook Could you confirm #157 works as expected in your case?

[kumabook]

@ishkawa Thank a lot! I confirmed. It worked perfectly fine for me! And the implementation looks more swifty!