Rename `XCDYouTubeVideoClient` into `XCDYouTubeClient`
API: Give the possibility to cancel the video information retrieval operation
Rename `getYouTubeVideoWithIdentifier` method into `getVideoWithIdentifier`
Remove the +[XCDYouTubeClient sharedClient] method
Add language identifier initializer and @property to XCDYouTubeClient