[Fixes #79] Work with non zero tmux base-index.
[Closes #45, #36] Add support for rbenv.
Merged iNecas:multi-tile by hiroara

Set layout after adding each new pane

Especially when using tiled layout, creating more panes ends up with message:
"pane too small". For example:

    tabs:
      - shells:
          layout: tiled
          panes:
            - echo 1
            - echo 2
            - echo 3
            - echo 4
            - echo 5
            - echo 6

It's because the layout is set after creating all the panes. Setting layout
after creating each pane fixes this issue.