2.1.5:
  - added Session.use_spawn=, AbstractSession.use_spawn=, and an :use_session=>
    option to AbstractSession#initialize.  if any of them are set the code uses
    Spawn::spawn to create external processes instead of Open3::popen3.
    Spawn::spawn uses named pipes (fifos) for IPC instead of forking and pipes.
    the fork used in popen3 can cause strange behaviour with multi-threaded apps
    (like a tk app).  see source for details

2.1.4:
  - added Thread.exclusive{} wrapper when io is read to works in multi
    threaded apps