Add library app (fix #7 #10)

Ref: #7 #10

This only adds the basic logic of the app. More to come in other PRs :)
Probably unrelated to this specific change, but urlpatterns should be removed in favor of a simple list of url since:
- it is not used
- it will get removed for Django 2.0
More reference at https://docs.djangoproject.com/en/dev/ref/urls/#patterns