v0.8.0

• #46 Change license from BSD to MIT
• Move to new organization, tmux-python
• Support package updates to pytest, sphinx, etc.
• Travis/CI: Limit tests to Python 2.7 and 3.6 (removed 3.3 to 3.5)
• Travis/CI: Update pypy veersions
• #103 Server.new_session learned how to run commands in
  window on session start, thanks @grimpy!
• #68 Make Server.has_session() use returncode, thanks
  @jlargentaye! This should make has_session more robust.