Import list of Git screencasts from gitcasts.com

As part of consolidation efforts, mentioned at [Training sites merger](https://github.com/github/training.github.com/issues/62), this brings in the gitcasts.com page which listed the screencasts posted at youtube.com/github

By default it embeds the first lesson onto the page.

To keep from touching more HTML templates, the page generation has been thrown into the `rake` process, sloppily cribbed from the regular page generation method.

Not having done CoffeeScript before, as suggested on [the style guide](https://github.com/styleguide/javascript "there are no events"), this also includes some bare JavaScript and jQuery to catch those links and load up the video on the page so the user doesn't have to go away and can follow through the videos in the order listed.