Migrate previous "os clean install" md

Exploded as a TODO.md + a First-run.md, that will probably be condensed
in the README.
Add Gimp
Move Sublime Text message only if needed
Add futur stuff for Alfred.app
Add Transmission.app prefs
export EDITOR
Add a lot of osx defaults
OS recognition for futur Linux* usage
Add functions in every bin to avoid error when making standalone call