[coffeescript/en][WIP] Rewrite

So, as discussed in #387, I'm rewriting the coffeescript tour. I'm creating this PR for the purpose of tracking progress and collecting feedback.

My goals are:

1. Spend most of the time explaining the *differences* from JS. The JS tour already goes into enough detail about things like prototype inheritance, there's no point duplicating that effort here.
2. Try to be more concise than the official CoffeeScript.org page - once again, no point duplicating that here. In particular, that means not explaining what `coffee` compiles something down to. (The part I've written so far, the first section, doesn't really achieve that, but that's because that section is trying to be a bit introductory as well.)

Feedback is welcome, of course - that's the point of this PR.

(Those of you who also have a commit bit: **please don't merge this yet!** I'll merge it once it's done and everyone's happy with it.)