[#47] Initial integration with Papertrail - Articles now have versions

* Article#versions for an array of ArticleVersion objects
* ArticleVersion table for just the article versions
[#47] Ignore #access_count when detecting for changes to Article.

We dont' want to create new versions every time the article is accessed.
[#47] Adds Article#status to ArticleVersion

This lets you pick versions depending on their status.
[#47] Use the most recently published article on #show. Breaks preview!
[#47] WIP: Refactor QuickAnswersController; preview + versioning.