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. There's a lot I haven't done and the bits I have done I want to revisit too. You'll see I've got my little "TODO" list in the markdown too - I left that there to easily keep track, I'll obviously remove it once I think I'm fully done.

Thanks!

(Apologies for the large initial commit too - will be doing more frequent smaller commits from now on!)