Add Vienna (Opal MVC)

Vienna is the main MVC framework for Opal (http://opalrb.org)

Opal is getting some traction and Vienna has been run on production sites for more than a year now (millions of users per month).

Also has unit tests (via [RSpec](https://www.relishapp.com/rspec))

More about Opal: 
http://opal.github.io/opal-playground/
http://opal.github.io/opal-playground/rspec/
http://www.youtube.com/watch?v=GH9FAfKG-qY