For multiple files, only truncate filenames of schema objects. Fixes #61.

 * docs/dbtoyaml.rst: Point out only schema objects will be truncated.
 * pyrseas/dbobject/__init__.py (DbObject.extern_filename): Add
   truncate argument with default False.
   (DbSchemaObject.extern_filename): New method, which invokes super
   method with truncate=True.
 * tests/dbobject/test_extern_file.py: New test to cover change.