Auto create dirs and 12 factor app

I start refactoring code to allow running an instance of twitter-rss on Heroku. For this some changes are needed :
- Missing files and directories need to be created by the application
- following 12 factor app methodology (http://12factor.net/)

I also extract logic from flask application (which was no required but I did not manage to prevent myself to do it ;-) ).

A Heroku running instance is available here: http://fbo-twitter-rss.herokuapp.com/

Have a nice day !

Default settings are set in code, so we only need `settings.ini` file (which is stored on git) as a template for `local_settings.ini` file (which is not stored in git). This allow all developers to have their own local configuration (using environment variables or `local_settings.ini` file as they wish).