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
Possibly too complex a consideration for this discussion, but I gotta say it for peace of mind, in case this is a genuine error:

So having used my fair share of GC'd languages, this statement blew my mind. The idea that Ruby would *copy* the string was, frankly, insane. In every GC'd language I've used, I'd expect all references/copies of an object to really be a pointer to some fixed location in the heap. That is, the array's indices just contain a bunch of pointers to random locations in memory where Ruby decided to make the Strings. Thus, when you get `v[0]`, I would expect you to get a pointer to some random location on the heap *unrelated to the vector*. I would further expect it to be fine if the array reallocs behind your back, because *it doesn't actually store any of the strings*, but rather pointers to the constant location each string lives at.

Thankfully, Ruby's strings are mutable, and I could test this.

```
v = [];
v.unshift("Hello");

x = v[0];
v[0] << " everybody";
v.unshift("world");
v[1] << " in the club";
puts x
```

If Ruby actually copies the string, I can expect "Hello". If it behaves like C, I can expect "Hello everybody" (or vomit). If it behaves like other GC'd languages, I can expect "Hello everybody in the club".

Mercifully, it prints "Hello everybody in the club". Thus, the string isn't copied, it's just kept at a fixed location in memory that will only be deallocated when all references to it are destroyed, making dangling pointers impossible in the way I expect from GC'd languages.