add support for extended types: you can now pack/unpack custom python objects by subclassing Packer and Unpacker
kill some duplicate code from unpack/unpackb and move the logic to Unpacker.unpack_one. By doing this we no longer need to make the module-level pack/unpack parametric on the class, because they contain no logic at all
slightly change to API
automatically find the best format to encode extended types
add support to unpack all ext formats
implement Packer.pack_extended_type also in the cython version of the code
add the hook for unknown types also to the cython Packer
implement unpack_one also for the cython version, and add a test for it
add a test for unpacking extended types
implement unpacking for all the fixtext formats
implement unpacking of ext 8,16,32
add a note in the README