Change default demo video to a universally playable video
Add disabled `VCR_CASSETTES_DIRECTORY` environment variable
Add support for protected VEVO videos

iOS 7 and OS X 10.9 only

Fixes #11
Improve XCDYouTubeVideo -description and -debugDescription methods
Simplify boolean expressions
Handle protected VEVO video thumbnails

When getting video information from the webpage, thumbnail URLs are not available. Saving a half working XCDYouTubeVideo object (noStreamVideo) from the get_video_info response allows to reconstruct a full XCDYouTubeVideo object with title, thumbnail URLs and stream URLs.
Replace `id` with `instancetype` where appropriate
Refactor JavaScript signature unscrambling

* New XCDYouTubePlayerScript class for handling the JavaScript part
* Use the JavaScriptCore C API for iOS 5 and iOS 6 compatibility, see http://stackoverflow.com/questions/23514579/javascriptcore-framework-availability-on-ios