Wiki With Images Downloads & Folders

A draft implementation to support folder hierarchies and serving of raw files (images, PDFs, anything) in the Gollum wiki.

I'd appreciate your feedback in order to try and get this closer to a mergeable solution, fixing http://feedback.gitlab.com/forums/176466-general/suggestions/3935963-allow-wiki-pages-to-show-images and allowing the wiki to be a bit more useful.