Allow Inheritance of Resources

This allows to have resources with subclasses and still be able to `find_roles` or `roles_with`.

Rspec provided, all tests pass over here (both AR and mongoid).

The base scenario is this:

    class Organization
      include Mongoid::Document
      resourcify
    end

    class Company < Organization
    end

    user.add_role :owner, Company.create

before this commit, doing

    Organization.with_roles(:owner, user) #(same for find_roles)
    => [] 

  No more!! :)

  No interface were changed except that now resource classes have a `applied_roles` method with a children parameter set to `true` by default.

So doing
   
    Organization.applied_roles 
    => [:owner]

while

    Organization.applied_roles(false)  
    => []