Support for materialized views.

 * pyrseas/dbobject/schema.py (Schema.to_map): Add matviews to objects
   mapped.  (SchemaDict.link_refs): Link in materialized views.
 * pyrseas/dbobject/table.py (MaterializedView): New class for new
   functionality.  (ClassDict.query): Add relispopulated and adjust
   definition for materialized views.  (ClassDict._from_catalog):
   Process materialized views.  (ClassDict.from_map): Refactor
   handling of objtypes, and process materialized views.
 * pyrseas/testutils.py (PgTestDb.clear): Drop materialized views if
   present.
 * tests/dbobject/test_matview.py: Tests for new functionality.
 * tox.ini: Add pg93 environments.