support non-ERAV data models

[sheppard]

The current implementation relies heavily on vera. Generalize this to support three use cases:

• Populating a single table
• Populating an EAV structure (e.g. wq.db.patterns.annotate or any EAV implementation)
• Populating an ERAV structure (i.e. vera)

Obviously, only the ERAV structure will be able to ensure data can be imported without clobbering existing data in the database.

[sheppard]

This should be done after wq/wq.db#50. The ERAV use case can be generalized as a EAV structure with a foreign key to a NaturalKeyModel.

[sheppard]

Fixed with #5.