Allow the 'all_transitions' key in the 'on' clause

Not sure if this is a genera use case, but I've found myself trying to use a callback for all available transitions. Let's say, for example, I want to create a log for an entity, that keeps all the transitions it had made till its actual state. 

Thus, It would be nice to have something like:

``` yaml
callbacks:
   callbacks:
          after:
              log_activity:
                  on: 'all_transitions'
                  do: something
```

So, this is what I propose in this PR. :smile: