Allow schema-referencing foreign keys in sqlite backend

[fnimick]

This is implemented by stripping the foreign key target table schema reference, as sqlite syntax does not support it.

Foreign keys from a table not in a schema targeting a table in a schema, or vice versa, or foreign keys from a table in one schema targeting a table in a second schema are not supported by sqlite. An explicit error is raised in this case.

New Features

• allows the sqlite backend to create a table in a schema, with a foreign key to a table in the same schema

[fnimick]

In the latest commit, I've also updated the sqlite backend to explicitly fail on db.schema.table references as these are illegal under the sqlite parser. Let me know if you'd prefer me to move that to another PR.