Support Views and Materialized Views

[stevennic]

I find it convenient to declare my Views and Mat Views as Entities too. This allows ORM-based lookups and logical foreign keys against views to work seamlessly. However, if I set create_tables=True in generate_mapping, it attempts to create the Views as tables, and to create things like FK constraints against them, which leads to a DB error. It would be nice if we could flag an entity as a "non table" so it is exempt from some DDL, while still allowing it to behave as an entity. It would even be ok if we didn't try to stop the user from attempting to insert or update an entry as that would be a reasonable caveat, but at least it shouldn't prevent other entities from being created due to a DB exception on the views.

SqlAlchemy uses the __abstract__ directive for this purpose.