Add new delegate message: updater:willRelaunchApplication:withArguments

Adds a new message to the SUUpdaterDelegate, allowing the host app to handle launching of Autoupdate application. I use this to replace the call the [NSTask launchedTaskWithLaunchPath] with [NSWorkspace launchApplicationAtURL:options:configuration:error] so I can modify the environment in the spawned process. It also makes it possible to adjust the argument list, which can be useful when using a different relaunch path (a custom relaunch executable).