Enable Travis deployment for rap-website

[willbowditch]

• Added packrat for dependencies, seemed like the easiest way here without having to setup a Description file or long install scripts. Packrat has it's own issues, but probs the easiest way forward here.
• Travis will build the book every time a commit is made, but will only push the result to github when merged into master.
• The book is now pushed into a separate branch gh-pages which can be used to serve the website. Keep the code and results separate.
• Added a brief how-to in site/resources-deployment-ci-with-travis.Rmd this probably needs some edits!
• Moved the content into a subdir /site

[nacnudus]

Merged, thanks! :tada:

Except that for some reason GitHub won't actually merge it. I resolved the conflicts and pushed to master, and it thinks there's still a conflict here even though there clearly isn't. I'll just have to close it. Your commits are in master though.