Scopes

Address #20 by adding a new mixin for the parent model which defines `.with_state` and `.without_state` scope methods for the active record adapter. This implementation also requires that the parent model defines a `transition_class` method:

```ruby
class MyModel < ActiveRecord::Base
  include Statesman::Adapters::ActiveRecordModel

  #...

  private

  def transition_model
    MyTransition
  end
end
```
@alan could you give this a code review please?