1. Init
-------

  To wrap existing FDs pair or socket connected to SFTP server
  create new SftpClient object.
     
  SftpClient *sftp = new SftpClient(fdin, fdout);
     
  sftp -> connect();
  
  ... some work here ...
     
  delete sftp;
  

2. Synchronous requests (low-level API)  
---------------------------------------

  Below methods maps to related basic SFTP requests:
  
  SSH2_FXP_OPEN     |-> sftp -> open()
  SSH2_FXP_CLOSE    |-> sftp -> close()
  SSH2_FXP_READ     |-> sftp -> read()
  SSH2_FXP_WRITE    |-> sftp -> write()
  SSH2_FXP_OPENDIR  |-> sftp -> opendir()
  SSH2_FXP_READDIR  |-> sftp -> readiir()
  SSH2_FXP_REMOVE   |-> sftp -> remove()
  SSH2_FXP_MKDIR    |-> sftp -> mkdir()
  SSH2_FXP_RMDIR    |-> sftp -> rmdir()
  SSH2_FXP_STAT     |-> sftp -> stat()
  SSH2_FXP_RENAME   |-> sftp -> rename()
  
3. Asynchronous requests (high-level API)
-----------------------------------------

  There high-level API on top of basic SFTP requests from [2] to run SFTP job
  asynchronous in background threads.
  
  A. Download job: sftp -> downloadFile(..., callback)
  B. Upload job:   sftp -> uploadFile(..., callback)
  C. List job:     sftp -> listFiles(..., callback)
  
  Callback function is called when:
  
  - job changed its states (e.g. job finished or error occured)
  - new transfer statistics arrived (upload/download jobs)
  - new portion of file list arrived (list job)
  
  To cancel pending job use job -> cancel() method.
  
  WARNING: All SftpJob objects MUSTS be freed by job -> release() when no
  needed longer.