Disqus Single sign on

Single Sign On for Disqus commenting
PT story: https://www.pivotaltracker.com/story/show/73738898

added `DisqusHelper`: renders sso embed script for disqus
added view specs
added Feature/Settings for Disqus: `config/settings.yml`, `config/settings/*.yml'

refactored deprecated rspec syntax in view specs

demo on http://yaro.herokuapp.com
why not use the standard string interpolation?