Digital rights management update and Xcode 10 support

• AVURLAsset category methods expect a certificate URL instead of a license URL. The license URL was previously used to retrieve a certificate, which was misleading.
• AVURLAsset convenience constructors with options have been added, mostly for diagnostics purposes at the moment.
• SRGAkamaiToken has been made private. It namely makes little sense for client applications to retrieve the Akamai token directly, since proper handling requires a resource loader, which is why this library ultimately exists. Moreover, our standard Google Cast receiver now handles content protection directly. With no real use case for direct token retrieval, we decided it was simpler not to expose SRGAkamaiToken to clients anymore.
• The project can now be built with Xcode 10 and the iOS 12 SDK.