Added help popup

Resolves https://github.com/raimohanska/turtle-roy/issues/8.

I created a HTML page out of the README.md, and added a `Help` button next to the `Examples` button which shows the page in a popup. I left certain unnecessary parts out, such as the `Running` section.

Technically, this solution uses a slightly modified leanModal.js (http://leanmodal.finelysliced.com.au/), with added scrolling due to the length of the cheat sheet.

The HTML page could be generated automatically out of the README.md with a script, but as the README might contain unnecessary information for the user, I felt like it's better to extract the cheat sheet into its own page.