Use ruby mode for gemfiles and guardfiles.

Gemfile stores gem dependencies for projects, which are loaded with gem.
Guardfile defines operations when certain files are changed, such as
auto-compliling LaTeX projects when one of the files is changed.
Set up theme-changer.

Automatically select colour theme based on whether its daytime or
nighttime.
For now, set up to use solarized-light by day and solarized-dark by
night.
Very cool, now just to make terminator do the same...