Intro redux

Because my '30 minute intro' was originally a blog post, the tone was a bit too light. It also was written a long time ago, and deserves a bit of a refresher for modern Rust. now that my work on the Guide is wrapping up, I want to give it a quick re-write as well.

This is not yet done, but I'm submitting it for feedback so far. I'd really like some comments on the ownership part in particular, which gets lower level than before, but is not strictly 100% accurate. Trying to strike a balance.

In general, I'm not sure I go into enough detail for those without systems experience, but am afraid of too much detail for those that do.

Rendered view: https://github.com/steveklabnik/rust/blob/intro_redux/src/doc/intro.md

/cc @wycats @nikomatsakis @brson etc
Is it worth the time/space to explain it in that much detail? It seems like a bit of an implementation detail of the package manager (not even the language itself) that would be better explained here as something like "Cargo.lock ensures records the versions of any dependencies, for repeatable builds; you can explicitly move to a newer version of a dependency with `cargo update`", with a reference to the cargo docs for more details about how it works.