Add schema option and use when creating sqlalchemy Table instance

[JohnCohorn]

Needed a 'schema' option when configuring a foreign table when the remote database(Postgres) organized its tables into schemas. If tables are not qualified with the schema name they cannot be accessed if they are under a schema and it did not appear possible to specify a default schema via the db_url. Hopefully this or a similar schema option can be added.

[rdunklau]

Thank you, I just merged it: https://github.com/Kozea/Multicorn/commit/636f63b764f8691a3b0ba9703f0a73fd112a7b32