1) Divided readonly and system flags of Field model. Readonly flag only restricts insert operations, while system flag restricts also create and drop table operations
2) Added Merge engine and tests for it
3) Added docs for Merge engine
4) Added opportunity to make Field readonly. This is useful for "virtual" columns (https://clickhouse.yandex/docs/en/single/index.html#virtual-columns)
- Querysets no longer have a default order when `order_by` is not called
- Added `autocreate` flag to database initializer
- Fix for SELECT FROM JOIN (#37)