Debug and set tmux options and env see rbenv_sample.yml

Allows setting various tmux options and env see rbenv_sample.yml.
Also gives devs a copy of what was executed to help debug.  this script can also double as a fallback in cases where the ruby exec is not quite the same as running a script from your shell.
would close issue #39

 just to clarify, this is not an end point but a step along a path.
The end point:

 - You can set shell environment variables per tmuxinator tab
 - You can set tmux session and tmux window/panel options per tmuxinator tab
 - You expose yourself to those settings by switching (`alt+N` to switch to the tab `N`) to that tabs tmux session
 - **This is important**:In all sessions, i.e. whenever you switch (`alt+N` to switch to the tab `N`), you still have a synchronized view of all the tabs (and their panels), all that is different is the shell env and tmux option settings you wanted to be specific to that tab, e.g. various build and test env variables such as `PADRINO_ENV=test` in one tab and `PADRINO_ENV=development` in another tab, etc. etc.

What changes from current behavior is that now to exit you do `ctl+a:kill-session`, if my changes get accepted you would `ctl+a:kill:server`, or you could write a more sophisticated attach logic.

@aziz, @prognostikos your thoughts? 

I'm not going to go further until @prognostikos changes are pulled in since I don't want to do all this and find its not the `right thing` for where tmuxinator is headed - this has happened to me with Bundler and a stack of git support changes I made, so I won't repeat the mistake here :)

HTH