Initial submission of support for yamltodb --revert option.

 * docs/yamltodb.rst: Document new option.
 * pyrseas/database.py (Database.diff_map): If revert option selected,
   swap the top level database objects.
 * pyrseas/dbobject/column.py (Column.diff_map): Check that statistics
   attribute exists before determining what SQL to generate.
 * pyrseas/dbobject/schema.py (SchemaDict.diff_map): Add special case
   for pg_catalog schema.
 * pyrseas/testutils.py (InputMapToSqlTestCase.to_sql): Add revert
   argument and pass it to config_options.
 * pyrseas/yamltodb.py: Add --revert option.
 * tests/dbobject/test_schema.py (SchemaUndoSqlTestCase): New test
   group to verify basic undo processing.