Scopes based on states names for ActiveRecord

I added module that automatically generate ActiveRecord scopes based on workflow states.
The name of each generated scope will be something like `with_<state_name>_state`

Example:

    class Order < ActiveRecord::Base
      include Workflow
      workflow do
        state :approved
        state :pending
      end
    end

    # returns all orders with `approved` state
    Order.with_approved_state

    # returns all orders with `pending` state
    Order.with_pending_state

How do you like it?