2.1.6:
  - wrapped send_command in a Thread (send async) so output processing can
    commend immeadiately.  this was o.k. before, but had strange behaviour when
    using popen3 from threads.  thanks to tanaka akira for this suggestion.
  - iff ENV['SESSION_USE_SPAWN'] is set Session uses Spawn::spawn instead of
    Open3::popen3.  also noted that spawn seems to be a bit faster.
  - added tests for threads.
  - run 'sh SESSION_USE_SPAWN=1 ruby test/session.rb' to test using spawn
  - added test for idl so it's test is not run if system doesn't have it, all
    that should be required for 'ruby test/session.rb' is should be sh'
  - removed sample/tcsh and note about Tcsh and Csh in README - stderr
    redirection/separation is flaky in those shells
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