context.events should use dot notation

Ideally we'd be able to do something like this:

``` python
event_names = [event.action.name for event in events]
if 'Approve Taxes' in event_names and 'Approve Parking' in event_names:
change_state('Approved')
```

But currently we have to use:

``` python
event_names = [event['action']['name'] for event in events]
```

Not a huge deal, just easier to write.
Related to #20

No file, test, or entry point is named. Start by tracing how context.events is constructed and consumed, then identify the existing tests for event access; done means supporting the shown dot-notation example without breaking current dictionary-style access.

索引モデルが issue の本文から書いたものです。