• Save EAGAIN errors after every libssh2 channel and session
  call.
  
• Extend exit_signal method to return the tree values comming
  back from libssh2_channel_exit_signal.
  
• Make Channel blocking method be just a wrapper for session
  blocking method.
  
• Add channel wait_eof method.
  
• Make exit_signal and exit_status call wait_closed under the
  hood, otherwise they could return a wrong result.
  
• Make wait_closed call wait_eof under the hood to avoid bad
  usage errors.
  
• Add method exit_signal_number that translates the result of
  exit_signal to the matching local signal number (note that
  on the remote machine the signal number may be different).
  
• Channel CLOSE now mimics real file handles better and sets
  $?.
  
• Improve the test script so that it can be better automated.