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 !

This setting is used by `Tweet.to_jinja2()` method in `twitter_rss.py` line 105. But i do not known it's purpose.