[importer] Implement SourceFile#target_schema
[importer] Set schema at initialization time for Column
[importer] Set schema at initialization time for Indexer
[importer] Indexer does nothing if no the_geom in table
[importer] Implement Loader#valid_table_names
[importer] Implement ShpLoader#valid_table_names
[importer] Make runner support files that produce several tables