Add library app (fix #7 #10)

Ref: #7 #10

This only adds the basic logic of the app. More to come in other PRs :)
Why not use a Queryset for that ? We now can use the power of Queryset::as_amanger (https://docs.djangoproject.com/en/1.7/topics/db/managers/#create-manager-with-queryset-methods)