Guide macros and unsafe

The last two sections of the guide, and a small conclusion. I suck at conclusions.

I also realized I never covered strings, so I'm going to put that section up before we're actually 'done.'
Maybe this could be moved below the `extern crate regex` (and the attribute could be combined into a single line):

```rust
#![feature(phase)]

extern crate regex;
#[phase(plugin)] extern crate regex_macros;
```

I think this may make the attributes seem a little less like magic, since as it stands they're currently just piled together and it's not immediately obvious that the `phase` one applies to the crate (the `!` distinction is relatively subtle for someone just learning the language).