Added tests for DirResource.
Filenames are rendered with a specialized template engine: FilenameEngine.
Simplified resource factories in tests: using default (empty) values of constructors.
Refs #19 - Moved 'render_filename()' to Resource class. Added 'filename_engine' attribute.
Refactored MockEngine.render() return value: just like engines.
Tested DirResource.render_tree(). Moved MockEngine to diecutter.engines.mock. Improved and tested MockEngine (using str.format in render result).
Added tests around diecutter.resources. Fixed regression in DirResource.render().
Fixed tests for Python-2.6