Wagtail-simple-gallery

New:
- Customized the admin view for images for better user experience.

Fix:
- Tweaked the gallery css a bit.

New:
- Added a caption (image title) when viewed via lightbox.

Fix:
- `image_limit` in `{% simple_gallery %}` tag had no effect if the number was a string.

Dropped the idea (and implementation) of using custom image models. Collections for the win!