Support reference entities with non-integer identifiers

Currently ER supports only entities with numeric IDs, but it fails for others - e.g. remote entities as implemented by fluxtwitter. I know there has been already an issue for that - but I've been unable to find it :-/

Attached patch quickly changes the column type to varchar - what makes it usable for other entity types. However, if you are open for doing this, I'd propose / roll a patch that defaults to integer but uses varchar if the entity property info of the target entity type's identifier is not 'integer'.