Rewriting the Backbone, AMD and RequireJS section

As discussed in #233, I've been taking a look this evening at the RequireJS section.

Please be aware that this very much a massive WIP. It's not all done at all but I wanted to make the PR early so you could see it as it was taking place, and also provide feedback along the way. 

I've rewritten bits but a lot of it has been more reworking, or moving certain paragraphs around.

As I said, hugely a WIP but would love to see your thoughts. I'll continue working on this this evening and over the next couple of days.

Thanks!