Adds a config setting for result image URL scheme

So, it turns out that some non-GitHub wikis (in my specific case, the gollum included in recent GitLab) don't handle the schemeless image URLs for annotated results very well. Or, really, at all.

This adds an image_scheme config option so those of us using GitHub alternatives can still see our results all pretty like.

Note: This also includes the "project_dir can't be private" commit requested for merge in hybridgroup/gitnesse#28