Remove duplication from adding transition logic

Removes the duplication in statesman/machine.rb.

@isaacseymour @mrappleton what do you think?
I would prefer:

```ruby
options.merge(callback_class: Callback, callback_type: :before)
```

as it gets rid of a pair of curly braces, and prevents someone a bit mental from doing something like:

```ruby
before_transition from: :state1, to: state2, callback_class: MyHorribleHack
```

(this applies to other uses of `add_callback`)