Guide: Variable bindings

Whew. So much here! Feedback very welcome.

This is the first part where we actually start learning things. I'd like to think I struck a good balance of explaining enough details, without getting too bogged down, and without being confusing... but of course I'd think that. :wink: 

As I mention in the commit comment, We probably want to move the guessing game to the rust-lang org, rather than just having it on my GitHub. Or, I could put the code inline. I think it'd be neat to have it as a project, so people can pull it down with Cargo. Until we make that decision, I'll just leave this here.
> I see the latter as dismissive of the reader, rather than respectful. But I suspect that others may think that an "aww shucks" tone it makes the text more approachable, 

This is a constant issue, which I'm not sure how to resolve. I had a really interesting, lengthy discussion with someone about "basically," the other day: I read it as synonymous with 'essentially,' and others read it as super condescending. 

Gimme a quick moment and I'll fix both of these. I _do_ think that searching on google for terms is a good idea, but I'd rather do that as part of a final editing pass than as I go, that's the kind of work that's nice to just do all at once.