[ASVideoPlayerNode] Added an optional full screen button

[MarvinNazari]

To enable the new control type ASVideoPlayerNodeControlTypeFullScreenButton you need replace the default default controls.

func videoPlayerNodeNeededDefaultControls(videoPlayer: ASVideoPlayerNode) -> [AnyObject] {
        return [
            NSNumber(unsignedInt: ASVideoPlayerNodeControlTypeElapsedText.rawValue),
            NSNumber(unsignedInt: ASVideoPlayerNodeControlTypeScrubber.rawValue),
            NSNumber(unsignedInt: ASVideoPlayerNodeControlTypeDurationText.rawValue),
            NSNumber(unsignedInt: ASVideoPlayerNodeControlTypeFullScreenButton.rawValue)
        ]
}

and even return an image for the button

func videoPlayerNodeFullScreenButtonImage(videoPlayer: ASVideoPlayerNode) -> UIImage {
    return UIImage(named: "video-player-fullscreen")!
}

[Adlai-Holler]

@MarvinNazari Thanks for adding this! Video player node is becoming very fleshed-out, and just in time since top-notch video support is highly sought-after by users this year.

[MarvinNazari]

@Adlai-Holler Happy to contribute, even with my limited Objective-C skills 😃