Add get_pty named argument to SSHClient.exec_command()

Reason to do this is that saying:
  sshCli = SSHClient()
  sshCli.connect(....)
  stdin,stdout,stderr = sshCli.exec_command("daemon_in_foreground",get_pty=True)
  stdin.write('\x03') #^C
  stdin.flush()

ends up killing the process as expected.