Openapi-sqlalchemy

- Add support for Python 3.6.
- Add connexion example application.
- Fixed bug where referencing a schema which uses allOf in many to one relationships does not merge the allOf statement.
- Fixed bug where a type hint that is not always exported from SQLAlchemy may cause an no member error.
- Add schema checking for extension properties.

- Add support for backref for many to one relationships.
- Refactor to remove reference resolving decorator.
- Add integration tests for major features.

- Add support for python 3.8.

- Update name from openapi-SQLAlchemy to OpenAPI-SQLAlchemy. All urls are expected to keep working.

- Add support for `allOf` for models.

- Refactor column factory to use fewer decorators.
- Change exceptions to include the schema name.
- Add support for `$ref` for models.