Libtmux

- 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.

- Port `retry` function from tmuxp
(<https://github.com/tmux-python/tmuxp/issues/354>)

- Don't add -x/-y in tmux >= 2.6 if running tmuxp from inside
client.

- Allow `Window.select_layout` with no args
- Fix test where `bell-` was no longer ambiguous as of tmux 2.6

- Hotfix to support tmux 2.6 session creation

- 65 Add session id to commands, thanks [askedrelic]