Improve Thumber::create to allow thumbnails to be regenerated

A friend of mine asked me to help him out because he needed the `Thumber` addon to regenerate his thumbnails for him if his PDFs had been updated since the last time the thumbnails were created.

After implementing that functionality for him, I thought that others might be having this problem and that it would make a good core addition to the addon.

Attached is a patch that improves the logic for deciding when to create a thumbnail to the following:
- Create a thumbnail if no thumbnail exists
- Create a thumbnail if a thumbnail exists but it is OLDER than the source file