Add (id/ref) relationships between entities (tables)

I analyzed the schema, starting from names to make assumptions
about the relationships, then checking and analyzing data to
confirm my assumptions.

Sources:

* for the schema:
  https://datamarket.azure.com/dataset/mrc/microsoftacademic#schema

* for data [login required]:
  https://datamarket.azure.com/dataset/explore/fd118e3b-6451-4544-bb18-eb01f8970aa5