Add library app (fix #7 #10)

Ref: #7 #10

This only adds the basic logic of the app. More to come in other PRs :)
Humm, good point.

But I guess I can have more than one NULL value in the db, but I can't have twice "", right? Which I think is what I'll get from the form. So I need a specific clean on the form to map "" to None.