Move packers and unpackers out of 'include' directory (now in 'ext' directory), they don't belong to st_asio_wrapper library.
Add pingpong test.
Add streaming unpacker to receive native data.
Support non-copyable msgs.
Make asio_server and test_client more appropriate for performance tunning.

Move packers and unpackers out of 'include' directory (now in 'ext' directory), they don't belong to st_asio_wrapper library.

Version number not changed.

Force to use c++98 in compatible edition.
Fix bug: if receive buffer overflow and on_msg returns false (which means using receive buffer),
 st_socket will call on_msg again and again (asynchronously) for the same msg until receive buffer becomes available.