bugfix no 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