Add migration for ATEvent to DXEvent

@pbauer 

:information_source: :cold_sweat:
Since I'm not really familiar with DX, please review carefully.

Timezone will be set to `UTC` during migration, is this OK?

I'm using the `IEventAccessor` to set informations on the Event, expect for the `text` attribute. The migration of the `text` attribute is handled by the `DocumentMigrator`, but this sets the value directly (event.text = ...)

Further:
The assertion...
```python
self.assertEquals('Event', dx_event.__class__.__name__)
```
... is not active, because the Event klass name is `Item`. I guess this is wrong, since all other types has their own klass.