Change critical CSS conditional logic

I'm updating my slides for [CSSConf](http://2014.cssconf.eu/) this week, and needed the ability to turn off CSS being inlined into the `<head>` of the document to generate a speed comparison. We current disable it by default in `Play.isDev()` mode but cannot in production. Therefore, this PR does two things:

- Rename the switch to be more explicit of its intentions
- Add the switch to the conditional logic surrounding the inline process.

This will enable me to turn off the switch in CODE and use webpagetest.org to generate this [timeline video](https://speakerdeck.com/patrickhamann/css-and-the-critical-path-cssconf-may-2014?slide=32) again.
This is the main line of the PR.