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.
... what I mean is that I suggest that this is not entirely true, and that we'll talk about how Rust infers if this variable is used later. My plan is to show something not-quite-as-ridiculous as the case you linked to, once we cover if statements, initializing it in the `if`.

Maybe that is too far out and I should just add this here. I wasn't sure, so I shipped it. :smile: yay feedback!

What do you think?