Threading done

It's been a long week, but the conversion to Cocoa threads is basically done.  The Downloader class has been rewritten, the new Downloader3 class uses NSUrlDownload instead of python libraries.  Among other things, this will allow downloads to become resumable.  Before I tackle this, however, my next task is to change from storing data in ~/.maclibre to ~/Library, by (for example) using NSUserDefaults to store preference info.