Installation wizard

Welcome page should display information about how to setup the app: how to setup Twitter, and generate the OAuth token.

What I see is:
* First page: explain how to add the app to twitter
* Enter the app's key (if Python can't change settings.py, point the user to it)
* Explain what's up with authentification, choose between callback (if app is visible) and verification code
* Twitter's OAuth page
* Callback to the app (or enter verification code)
* Enter the user's key (if Python can't change settings.py, point the user to it)
* Check configuration & 'get started' page