[DONTMERGE] Interval Support

[Rudi3]

Should fix https://github.com/SeaQL/sea-orm/issues/426

I'm not sure what entitiy_model.rs does, so I'm not sure if i got that right.

(i got an error in combine.rs and just added the todo!s for now...)

This depends on changes in sea-query, so merging doesn't make sense atm.

[tyt2y3]

It looks great, although I am not sure how should we expose this Postgres-only feature. The entity_model.rs is a derive macro, and the code you added converts Rust type (from the struct attribute) into Column type