Updated: base test to use __decompressors__ instead

The base test now check that compressions is empty and __decompressors__
have the test decompressor class
Updated: replace all_decompressors mechanism by builtin_decompressors

- Remove the metaclass and the all_decompressors mechanism
- Add the builin_decompressors mechanism which knows only the
  decompressors provided by StreamDecompressor
- StreamDecompressor.open() now use by default only builtin
  decompressors without specific priority
- Moved priority mechanism from Archive class to Guesser class
Updated: recursion mechanism of guesser.py to go through ArchivePack

Instead of stopping detection when an ArchivePack is found, the Guesser
will try to guess other decompressors (so, it *excludes* the last
decompressor itself).

This means that the ArchivePack stream should not be an empty stream but
the source stream itself.