Merge commit '2079f11057532a4f291af2b63ff955e81c20a952' into entidi
Merge branch 'entidi' into dev
Implement as an extension to silverstripe-carousel

silverstripe-carousel is already providing a lot of functionality needed
by the gallery: a custom tag in the CMS with fields reordering and some
custom settings.

Now silverstripe-gallery provides only the missing bits: thumbnails
support and the templates to render a gallery instead of a carousel.
doc: add usage section
Merge branch 'dev'