Stricter adherence to Jekyll best practices

This pull request moves the files within the repository around a bit to better follow Jekyll design patterns and best practices, but **does not make any changes to the content itself**. Specifically:

* Move plays from `_includes` to a "plays" collections
* Store play ID and title as YAML front matter. This prevents the title in the index, the number, and the title on the play itself from getting out of sync, and DRYs up the layout. Not to mention, by *separating presentation from content* we can expose the information in a machine readable format
* Creates `plays.json` which is an API of the plays
* Move 404.html from `_includes/404.md` to `404.md` and better leverage `404` layout
* Dynamically generate the "plays index" from the play titles and IDs
* Move pages from `_posts` to `pages`, store as pages rather than posts
* Move techfar from to its own layout (rather than stuffing the layout in a post and then including the content as a partial)
* Use `jekyll-redirect-from` to create a case-insensitive redirect, rather than maintaining two pages
* Remove the unused test and about pages as well as page layout.

Ideally, content should live in `.md` files in a logical folder structure with the presentation information living in `_layouts`, but with only two templates right now, index.html contains its own presentation information. A common header/footer may be a better long-term solution if there are going to be additional pages.

/cc @gbinal on the API and @parkr on the Jekyll best practices