Allows the target method to be aliased within a definition

In my current project, I've separated the finite machine from a host Job model using a FiniteMachine::Definition class. In writing the definition, it has struck me that it would be much more readable if I could alias the target method. Using the example from the README:

```ruby
class Engine < FiniteMachine::Definition
  initial :neutral

  alias_target :car

  events {
    event :forward, [:reverse, :neutral] => :one
    event :shift, :one => :two
    event :back,  [:neutral, :one] => :reverse
  }

  callbacks {
    on_enter :reverse do |event|
      car.turn_reverse_lights_on
    end

    on_exit :reverse do |event|
      car.turn_reverse_lights_off
    end
  }

  handlers {
    handle FiniteMachine::InvalidStateError do |exception| ... end
  }
end
```