v0.06

  - callback functions were made optional, as they are in the XS version
    (support for older versions of libarchive)
  - various fixes to support libarchive 2.x
    including versons of libarchive that do not have archive_read/write_free
    (use archive_read/write_finish instead)
  - fixed argument count bug that only manifested itself on FreeBSD (GH#13)
  - build fixes for NetBSD