Creates 'js/backbone/entities/header.coffee' and links it to 'index.html'

In an attempt to refactor our ‘HeaderApp.List.Controller’, we want to
improve the structure of our code by letting ‘entities’ to take
responsibility of handling models and collections, not controllers (as
it is now). This is because if someone was to maintain our code, they
would have to know all about our controller’s implementation to
understand that the header menu items are defined there. Also, not in
this case but possibly many others, entities of models and collections
may be needed from various places. By putting them in one very specific
place, it is difficult to reuse them.