Bulk support, ArchiveHandler

• Adds support for bulk file handling back
• Adds the first bulk handler, ArchiveHandler, for multi-file downloads

Note: Existing third-party handlers need to use the new BaseExport::getFile() method instead of referencing $file directly, because it no longer exists. Because the single-file restriction was an oversight originally, and because you haters don't use this package anyways, this BC will not merit a new major version.