Django-pg-returning

Bug fixes
1. Fixed django 2.2 compatibility

Testing
1. Added django 2.2 to travis

Features
1. Added bulk_create_returning method to QuerySet

Bug fixes
1. Fixed foreign relations returning issue (https://github.com/M1hacka/django-pg-returning/issues/10)
2. Fixed filtering with empty array in IN clause (https://github.com/M1hacka/django-pg-returning/issues/9)

Features
1. `first()` and `last()` methods for ReturningQuerySet
2. Created abstract `UpdateRetruningModel` to integrate with
3. `UpdateReturningModel.save_returning()` method

Features
1. Ability to create empty ReturningQuerySet
2. Ability to concatenate ReturnginQuerySet-s with same fields

Testing
Added PostgreSQL 10 to travis config

Bug fixes
Fixed bug when `only` or `defer` methods are not used and model has relation fields ([issue 2](https://github.com/M1hacka/django-pg-returning/issues/2))