Add Twitter Bootstrap to style the app

Add Twitter Bootstrap with default styling with the twitter-boostrap-rails
gem.
Establish the basic layout with a main content area and a sidebar
Add *new article* link, category list, and *about* text to the sidebar
Add top navigation bar with home link and a placeholder for logins

This adds a fixed, top navigation bar displaying the app name with a link to
the site root. There is also some placeholder text waiting for user sign up
and log in/out functionality to be added.
Improve support for responsive design and Internet Explorer