Add option to automatically retry races

When race conditions occur, Statesman raises a TransitionConflictError. The user then needs to handle that error.

In many cases we know up front what we want to do if there's a race: we want to retry the second action and get back the right error for it (or have it succeed). This PR adds a `retry_conflicts` method to `Machine` that allows us to specify that up-front:

```ruby
Machine.retry_conflicts { instance.transition_to(:new_state) }
```