Ownership guide

This is a work in progress, but this should get *extensive* review, so I'm putting it up early and often.

This is the start of a draft of the new 'ownership guide,' which explains ownership, borrowing, etc. I'm feeling better about this framing than last time's, but we'll see.
Shouldn't this be:

~~~rust
fn add_one(mut num: Box<int>) -> Box<int> {
    num += 1; num
}
~~~