Adding POSSE feature, first with hard coded Twitter support.

I know this lacks:
• UI (you need to setup using rails console first)
• Documentation (no one knows how to setup this but me)
• Specs (lots of them)
• Refactoring (I already know how to do it)

Reasons why I'm pushing a so ugly feature / code are simple:
• I need to play it in real life to fine tune the feature.
• If we wait for a feature to be perfect before we push it, we'll never push it

Hopefully, I have lots of time in a lost French countryside to polish it.