cubicweb #2719191 Have a demo mode "cubicweb-ctl run-server cubename" [open]

Have a oneline cubicweb command that runs a live server with an sqlite backend.

The curious user could then go :

• virtualenv cubicweb ; . cubicweb/bin/activate
• pip install cubicweb-blog
• cubicweb-ctl run-server blog
• point his browser to http://localhost:8080/