Use github-pages gem and Ubuntu 14.04 vagrant image.

The github-pages gem currently depends on:
- Jekyll 2.3.0
- Ruby 2.1.1

Now using Ubuntu 14.04 Trusty vagrant image from VagrantCloud.
Added default layout settings for pages and posts.
Use new blank layout for ATOM XML news feed.
Removed extra code snippet indents in guidelines.
Suggest --no-bin-links for 'npm install' for Windows hosts.