Nested directories and keeping generated files.

Two changes include in the pull request:
1. Add ** in Dir.glob for allowing nested subdirectories in galleries. In the gallery tag need to specify sub-path of the images as in:

```
{% gallery new_gallery %}
new_gallery/IMG_1738.jpg:: Comment
new_gallery/IMG_1739.jpg:: Comment
{% endgallery %}
```

2. Keep generated thumbnail files from overwriting.

When plugin is used to generate content in `_site` subdirectory, then all generated file *-thumb.jpg are deleted right after generation. Adding them to `site.static_files` prevent form cleaning.