Allow to define the data schema in Avram :: Model

My suggestion is to allow the choice of schema in the application data model, so it will be possible to work with different schemas from the standard postgresql public.

The issue does not name files or tests. Start by locating Avram's model and PostgreSQL connection or schema configuration, then compare how migrations select the schema. Done means an application model can select a non-public schema and the behavior is covered by tests.

Written by the indexing model from the issue text.