Add quick install script, tweak CSS, and provide a screenshot

I've added a "quick install" bash script that can be run multiple times over.  Installing can now be done with a single command (updated README accordingly).  If this colloquy style is already installed, then the install script pulls updates via git.  Otherwise the install script verifies git is available (if not it errors out), clones the repository, and handles the symlink for you.  The quick install script has been thoroughly tested.

As for CSS tweaks, I've updated the style so it uses the true font size that's set in the Preferences => Appearance settings.  The padding around the edges has been reduced a little which I feel looks a lot better (even though it's not exactly the same as Campfire)!  I've also added a new "No Events" variant style.  

Finally, I've added a screenshot and displayed it in the README.  Please see https://github.com/benlinton/colloquy-theme-campfire

FYI, once this pull request is merged in, you'll be able to close all currently open issues.