Feat/format warc

This changeset provides support for ISO 28500 archives (aka [The WARC File Format][1]), as produced for instance by `wget --warc-file=...`

The idea is to be able to use proven tools (i.e. libarchive) and a known workflow to operate on those archives.  

While there are dedicated tools ([warc-tools][2]), [hanzo-warc-tools][3], etc.) their usability is highly questionable: For example the author of this changeset couldn't decide between the 2 mentioned tool sets; the C variant looked a lot more robust and functional than the python variant and yet the C variant has been declared obsolete already, while the python tool set does not even offer a means to extract one particular file.

This changeset is by no means complete and fail-safe (especially not tested under non-unix environments) but it should give anyone who's interested a good starting point.

  [1]: http://bibnum.bnf.fr/warc/
  [2]: https://code.google.com/p/warc-tools/
  [3]: http://code.hanzoarchives.com/warc-tools
As I mentioned above, if a write request can't be handled, the archive_write_header() call should return either ARCHIVE_FAILED or ARCHIVE_WARN.  Libarchive users look for ARCHIVE_FAILED to tell them to not bother writing a file body if the file type cannot be handled.