Missing Exhibition IDs, Constituent IDs?

[george08]

Hello.

First, sorry about issuing those pull requests before I absorbed the README properly.

Second, we're wondering if you have plans to add Exhibition IDs, Constituent IDs across the Exhibitions dataset? We're ingesting everything into a new database over here, and wondering how to handle missing things if we ever need to do an update (and there's a new row or two to integrate, but they don't have IDs...).