Replace scopes with class methods

In Rails 4 scopes do not work on abstract classes at the current moment.
You can read more here: https://github.com/rails/rails/issues/10658
In order the gem to work on Rails 4, replace the scopes with class
methods which should keep the same behaviour.
Merge pull request #2 from notonthehighstreet/vn/rails4_scopes

Replace scopes with class methods