switch from JTar to Commons Compress

JTar does not support long file paths (> 99 characters), which was breaking my application. So I replaced JTar with Apache Commons Compress, which supports both GNU and PAX long names. Also provided the `format` argument to `tarfile.open` to explicitly select PAX format (GNU would work as well).

I pruned down Commons Compress to only the sources necessary to handle tar archives. I did not modify any Compress code, but only removed files which were not imported by other files.