Release v0.9.0

[internal]: Renamed config.expandpath to config.expandshell.
[internal] [tests]: compat 2.7/3.3 wrapper for EnvironmentVarGuard for testing.
[shell_command_before]: You can now use environment variables inside of start_directory, before_script, shell_command_before, session_name and window_name.
[docs] [examples]: add example for environmental variables, examples/env-variables.json and examples/env-variables.yaml.
[internal] de-vendorize colorama. Thanks @marbu. #110
[tests] fix failure of test_pane_order on fedora machines #109 from @marbu
[manuals]: append .txt extension to manuals (repo ony) #105 from @yegortimoshenko.
[internal] Fix Server.attached_sessions return type #107 by @thomasballinger.
[internal]: update travis to use new tmux git repository.