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.
I'm new to Rust, but wondering: can you pass in a `Box<int> blah` to add_one like this?   These are the small details that are gotchas to me.   In particular, `String`, `&str`, `&String`, etc. etc. etc.

I think using Strings as an illustration would be very helpful.  Most people coming to Rust are likely to deal with strings first, and that is the first area where everyone gets really confused.