Private Event Layout Support

Conversation with @johndbritton precipitated this effort to support private event registration pages via the Training.GitHub.com page.

The layout is just a small tweak for the existing ones, with the exception of the event content. `markdownify` is run against any `page.content` should the event description need to support bullets or other Markdown formatting.

A root directory redirect was also setup to go to the in-person training page.

Each private event page is simply generated with a Markdown file in the `private/` folder and with the desired YAML frontmatter.

![screen shot 2013-10-17 at 2 30 09 pm](https://f.cloud.github.com/assets/352082/1355665/f23e9512-376a-11e3-9120-1404a2d3cf28.png)