Picks up SASS/CSS settings from /config/nembrot.yml.

Variables are picked up from /config/nembrot.yml via the Settings class
and the settings_styling method defines in /lib/sass.rb. This enables us
to use common settings in css and images (and snap images to the
baseline, for instance), and write out the colophon details.
Fixes tests.
Updates gems.
Removes therubyracer, adds better_errors gem.
Fixes reference to cut_image_binary_path.
Tidies up formatting.