Renamed Migrator to DbMigrator.
"He told me you killed my PK!" "No, I am your PK" (Added ReferencedKey API to allow specification of non-primary principal key)

This is the counterpoint to the ForeignKey API and is used to specify the key properties at the principal end of the relationship.

Note that Migrations doesn't support yet creating the required candidate keys so functional testing is limited to in-memory. Also, we decided this morning to remove the parameterless overloads of ForeignKey amd ReferencedKey, but I will do that as a separate change.
That name no longer has any meaning for me... (Remove parameterless overloads of ForeignKey and ReferencedKey)

After discussion of this API we decided that the semantics of these overloads were ambiguous as to whether they were for just specifying the end or indicating that the FK should be created explicitly in shadow state. Also, it was not super-clear which was the principal/dependent when no property was supplied. Given that the only real value is to avoid having to be explicit about either the FK or PK in cases where the principal/dependent need to be flipped, we decided it was better to remove these overloads.