2.1.8:
  - greatly simplified read loop using two reader threads, one for stderr and
    one for stdout alongside a mutex to protect data.  this streamlined the code
    alot vs. the old select method including allowing removal of the linbuffer
    class.  the interface remains exactly as before however.
2.1.7:
  - improved thread safe non-blocking read method
  - gemspec 
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 tes