Adds accessor method prefixes

- Adds prefix methods for model accessors
- Implements array setters
- Implements << setters

    class MyModel
      extend Bitmasker::Model

      has_bitmask_attributes :roles, 'has_role_' do |config|
        config.attribute :admin, 1 << 0
        config.attribute :manager, 1 << 1
     end
    end

    my_model.roles << :admin
    my_model.roles = [:admin, :manager]

    my_model.has_role_admin? # true