Add functional test for --directory input/output scheme.

 * pyrseas/testutils.py (DbMigrateTestCase.remove_tmpfiles): Add
   removal of two-level directory tree.
   (DbMigrateTestCase.{create_yaml_dir, migrate_target_dir}): New
   methods to run dbtoyaml and yamltodb with -d option.
 * tests/functional/test_autodoc_dir.py: New functional test based on
   test_autodoc.py.