Switch image URIs to HTTPS by default

This commit addresses #29

Rather than schemaless URIs (which break on non-GitHub wikis), use HTTPS as
default image URI format. This also prevents insecure content warnings that
would possibly show up if we used HTTP as the default.

Thanks to @ess for the issue/fix
Make Gitnesse::DirManager.project_dir public

This commit addresses #27

DirManager.project_dir is currently called from

- lib/gitnesse/hooks.rb
- lib/gitnesse/cli/helpers/wiki_helpers.rb

And there's no real reason for it to be private right now.

Thanks to @ess for the issue/fix