Rails 4 Support

Removes attr_accessible from model (since this responsibility has been
moved to controllers on rails 4)
Removes tests related to attr_accessible.
It's not a problem at all. Since the app doesn't expose the model over an
controller action, it'll be untouchable.

Changes model to use rails 4 style on using
where(...).first_or_initialize instead of find_or_initialize
Updates changelog

Now the changelog contains the rails 4 new support alert
Merge pull request #9 from filipechagas/rails4_support

Rails 4 Support