Inconsistency in thumbnail image path

[brian-rose]

In the current template, the thumbnail image is in a thumbnails/ subdirectory, but the _gallery_info.yml specifies the wrong path:

thumbnail: thumbnail.png

This causes the gallery generator build to fail for any cookbook that follows this pattern. An example of this was discovered in #29

[brian-rose]

Solution: we need to choose either to keep the thumbnail image at the root of the repo, or (probably cleaner design) use

thumbnail: thumbnails/thumbnail.png

in the _gallery_info.yml