bugfix auth_banner with bytes<->string

"Can't convert 'bytes' object to str implicitly" when using sftp

Unknown exception: Can't convert 'bytes' object to str implicitly
Traceback (most recent call last):
  File "/usr/lib/python3.3/site-packages/paramiko/transport.py", line 1601, in run
    self.auth_handler._handler_table[ptype](self.auth_handler, m)
  File "/usr/lib/python3.3/site-packages/paramiko/auth_handler.py", line 393, in _parse_userauth_banner
    self.transport._log(INFO, 'Auth banner: ' + banner)
TypeError: Can't convert 'bytes' object to str implicitly


EDIT: Maybe the following can cause problems too::

$ egrep -R "_log\(.*\+.*\)" *
paramiko/transport.py: self._log(ERROR, 'Exception: ' + str(e))
paramiko/transport.py: self._log(ERROR, 'Socket exception: ' + emsg)
paramiko/transport.py: self._log(ERROR, 'Unknown exception: ' + str(e))
paramiko/transport.py: self._log(DEBUG, 'Banner: ' + buf.decode())
paramiko/transport.py: self._log(DEBUG, 'kex algos:' + str(kex_algo_list) + ' server key:' + str(server_key_algo_list) + \
paramiko/transport.py: self._log(DEBUG, 'Debug msg: ' + util.safe_string(msg))
paramiko/auth_handler.py: self.transport._log(DEBUG, 'Methods: ' + str(authlist))
paramiko/auth_handler.py: self.transport._log(DEBUG, 'Allowed methods: ' + str(authlist))
paramiko/sftp_server.py: super(SFTPServer, self)._log(level, "[chan " + self.sock.get_name() + "] " + m)
paramiko/sftp_server.py: super(SFTPServer, self)._log(level, "[chan " + self.sock.get_name() + "] " + msg)
paramiko/sftp_server.py: self._log(DEBUG, 'Exception on channel: ' + str(e))
paramiko/sftp_server.py: self._log(DEBUG, 'Exception in server processing: ' + str(e))
paramiko/sftp_client.py: super(SFTPClient, self)._log(level, "[chan %s] " + m, *([ self.sock.get_name() ] + list(args)))
paramiko/sftp_client.py: super(SFTPClient, self)._log(level, "[chan %s] " + msg, *([ self.sock.get_name() ] + list(args)))
$