Allow multiple child objects

This is used as follows:

    # classes

    class User < ActiveRecord::Base
      acts_as_superclass of: {
        student: {type: :one},
        others:  {type: :many},
      }
    end

    class Student < ActiveRecord::Base
      acts_as :user
    end

    class Other < ActiveRecord::Base
      acts_as :user
    end

    # migrations

    create_table :users do |t|
      # ...
      # no fields necessary for the base class
    end

    create_table :students, as_relation_subclass_of: :user do |t|
      # ...
      # indices are created automagically
    end

    create_table :others, as_relation_subclass_of: :user do |t|
      # ...
      # indices are created automagically
    end

    # usage

    user = User.find 20
    user.as_student		# returns associated student object
    user.as_others		# returns array of associated "other" objects
    user.specific		# returns array of all child objects

    other = Other.find 6
    other.as_user		# returns parent object