Djangoevents

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- Renamed `Event.schema_version` to `Event.version`.
- Started setting `Event.version` based on schema file with the highest number.
- Added event version to stored event envelope under the `event_version` key.
- Added setting to add event version to event data as well (see "Event version" section in README).

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- Started automatically choosing Avro schemas with the highest version
(based on file names on disk).

- Changed way how event type string is computed. For example,
given User aggregate and Created event, now it would be "user_created"
(while in older versions that would be simply "Created" which sometimes
is not sufficient enough). Moreover it is now possible to override that
with `event_type` attribute set on an event class.

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- Made `DomainEvent.metadata` optional during serialization ([PR 10](https://github.com/ApplauseOSS/djangoevents/pull/10)).

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- Fixed exception when DomainEvent.metadata is not defined on saving to event store. [PR7](https://github.com/ApplauseOSS/djangoevents/pull/7)

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- When attempt to create an aggregate using ID of existing one,
`esdjango.exceptions.AlreadyExists` error would be raised.
NOTE: This is possibly a breaking change: previously Django's
standard IntegrityError was raised.