Cq

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- Fixed `Repository.get_aggregate` method (it now properly raises `DoesNotExist` error if there were
no events yet for given `aggregate_id`)
- Added `Repository.get_aggregate_or_None` method

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- Allow to register handlers for all events or all events for certain aggregate type

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- Fixed `Storage.gen_replay_events` (it was not yielding events)

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- Added ability to validate events
- Added `Storage.gen_replay_events` so user can i.e. follow progress of
events being replayed

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- Added ability to replay events (use `Storage.replay_events()` function)
- Added ability to upcast events
- `Storage.get_events` now accepts both `aggregate_type` and `aggregate_id`
- Renamed `cq.handlers.publish` to `cq.handlers.handle_event`
- Repositories can now be declared using `repos` dictionary at the `BaseApp` subclasses
- Implemented `Storage.iter_all_events` method