Add start_directory to support to new sessions and panes

[gandelman-a]

This allows specifying start_directory for creation of both
session and panes.

Clarifies some docstrings as well.

[codecov-io]

Current coverage is 78.25% (diff: 66.66%)

Merging #19 into master will decrease coverage by 0.06%

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[tony]

Looks good to me, 👍

[tony]

@gandelman-a do you need this published ASAP? I can tag a new version w/ this PR

[gandelman-a]

Hi Tony, no.. thats fine. Just setting up some scripts for my dev. env. and running from master so its fine. Thanks!

[tony]

Understood. This got released in 0.6.1 too

[tony]

thanks for the contribution again @gandelman-a ! nicely done!

[haakenlid]

tmux version 1.8 doesn't support the tmux new-session -c <start_directory> option. This caused an error in the tmuxp tests.

https://travis-ci.org/tony/tmuxp/jobs/190761397#L917
E LibTmuxException: [u'tmux: unknown option -- c', u'usage: new-session [-AdDP] [-F format] [-n window-name] [-s session-name] [-t target-session] [-x width] [-y height] [command]']

A possible workaround is to instead use default-path in tmux 1.8 (not supported in later versions) or to use subprocess to run something like this (cd $start_directory; tmux new-session -t $session_name), which should work in all versions.