- Do not use a property for a local variable.
- Use new-style class.
- Stop using logger.
- Save a few getattr calls.
- Do not keep references to buildout & options outside of __init__ .
- Convert mode in __init__ to detect errors early.
- Factorise several accesses (buildout['buildout'], properties)
- Drop redundant "cache" download.Download parameter: it is automatically
  fetched from first parameter internally.
- "Backward compatibility with other recipes" is just not "backward".
- Return downloaded file path, not just the optionally-created directory.