When diffing all apps, show tables not defined by a model

When migrating from a legacy schema to Django, sqldiff helps understand
what work remains. There may be entire database tables that exist and
are not defined by a model. Added support to sqldif to find these
tables. The tables are only checked when diffing all Django apps.
Merge pull request #552 from jdufresne/master

When diffing all apps, show tables not defined by a model