Please make EntityClasses and ExportConstants more extensible

[Tom Johnson]

We need a way to include our custom objects to BigQuery, I tried using the DonutsConfigModule to do this but ran into Blaze issues with circular dependencies. There is a thread on this with Ben, Nick and I making comments if you need more detail on this: https://groups.google.com/forum/#!topic/nomulus-discuss/pW9zyt3KUOA