Support native data types for abstractions.

[mgurley]

No description provided.

[mgurley]

Abstractor stores everything as a string in the abstractor_abstractions.value field. Ideally we would like to also support a native representation of the value as well. For example, date fields stored as dates, number values stored as numbers. A native representation will prove useful when you try to mix abstracted data with non-abstracted data. For example, the diagnosis date on an inside pathology case and a diagnosis date abstracted on an outside pathology case.