Remove the `IPHONE_SIMULATOR_CLASS` hack
Test `XCDYouTubeClient` instead of `XCDYouTubeVideoPlayerViewController`
Add unit test for using XCDYouTubeClient on a non-main thread
Make sure that XCDYouTubeClient works on any thread
Remove check for main thread

Now that XCDYouTubeClient supports starting an operation on any thread this check becomes redundant.
Add stream URLs to -[XCDYouTubeVideo debugDescription]
Move error related constants into XCDYouTubeVideoErrorDomain
Do not construct dummy error
Fix missing #import
Assert that the completion handler is called on the main thread
More responsibility splitting

* XCDYouTubeVideoOperation (NSOperation subclass) is now responsible for getting the data from the YouTube API
* #import cleanup
Add new `+defaultClient` method to XCDYouTubeClient
Make the video operation property weak instead of strong
Fix #imports in unit tests
Add unit test for canceling a video operation