Clarify class constructor usage for L.Class

Hey! I've been working on an app with Leaflet, and have been recently working with `L.Class` to encapsulate some of my Leaflet extensions. However, I noticed that there seems to be some behavior that is currently undocumented on the Leaflet API reference re: class constructors / the `initialize` method, so I wanted to add something about that.

Please let me know if my understanding of the `initialize` method is incorrect, so I can update this PR accordingly.

Edit: In addition, I updated the Gemfile with the [github-pages gem](https://github.com/github/pages-gem), which lets local development track the same [dependencies used in production on GitHub Pages.](https://pages.github.com/versions/)

Thanks for building an amazing map library!