Adds to_s to ensure better rb version compatibility
Doesn't unnecessarily overwrite existing thumbnails

By checking whether there's an existing thumbnail for each file
that is more recent than the file itself, we can keep old, useful
ones.
Merge pull request #6 from matthewowen/faster_generation

Doesn't unnecessarily overwrite existing thumbnails
Relicense to MIT
Merge branch 'master' of github.com:matthewowen/jekyll-slideshow