New API draft

Mostly for discussing before the new shared API (iOS + OS X) is finalized
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
Move XCDYouTubeVideo initializer into a private header
Use xcodebuild + xcpretty instead of xctool
Use `set -o pipefail` for xcodebuild + xcpretty