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.
You can actually do this

```rust
let x;

x = 2;
```

is fine. It may be worth rephrasing this to avoid implying that this isn't legal. [Recent example (ab)using this](http://www.reddit.com/r/rust/comments/293mhx/one_of_those_moments_where_you_stop_to_appreciate/).