Allow events to define a transition_missing hook.

I've added a `transition_missing` hook so that events can implement
their own logic for dealing with events received in an improper state.

For example:

```ruby
class Order
  stateflow do
    initial :received
    state   :received,:paid,:shipped,:cancelled

    event :payment_received do
      transitions from: :received, to: :paid
    end

    event :shipment_sent do
      transitions from: :paid, to: :shipped
    end

    event :cancel do
      transitions from: :received, to: :cancelled
      transitions from: :paid,     to: :cancelled

      transition_missing do |from_state|
        if from_state == 'shipped'
          raise "Can't cancel a shipped order."
        end
      end
    end
  end
end
```

This allows a user to define their own Exception to be raised, or,
in fact any logic (like send another event to move into an error state).