Forward autoincrement property from field to column

[smuething]

The autoincrement property of ormar fields does not get propagated to the SQLAlchemy column definition. As a result, the autoincrement property of the SQLAlchemy column defaults to auto, making it impossible to create DDL that does not auto-increment the database column.

With this small patch, it becomes possible to create a table with a manually managed, integer-based PK.