Fails to handle dependency on an existing table without a class

[eywalker]

Current implementation will fail if a newly defined table has a dependency on a table that already exists in a schema bound to a module, but does not have a corresponding class definition in the module. This is a rather convoluted, but certainly possible situation that must be addressed.

For a concrete example, if you are defining a new table called Experiment in module mouse that refers to another existing table called microscopes in the database mouse_setups that is already bound to a module called setups, as here:

definition = """
mouse.Experiment (manual)
-> setups.Microscopes

then this will fail to create an appropriate table relation representing setups.Microscopes.

[eywalker]

This is now more or less addressed via #52, and test cases are passing. Closing this issue.