Use autoloader for Archive

This makes life easier for downstreams who unbundle third party libraries. I've tested (in a very ugly and probably stupid way) that this will find Archive/Tar.php from OC's 3rdparty directory if it's there, or a system copy if OC's 3rdparty copy isn't there, on a test Fedora 20 instance.

Again, I'm just the monkey, I really don't know much about this stuff. All I know about OC's system of loading I learned today by bashing on it extremely inexpertly. Please check it carefully :)