New ownership guide

This is the start of the new "Ownership" guide, to replace the lifetimes guide.

I'm still working on it, but I wanted to share my progress, as this guide is _very_ important, and I'd like as many eyes on it as possible. :smile: 
There's a bigger problem with this statement: [it's wrong][1]. Can't use keywords as lifetime names.

[1]: http://play.rust-lang.org/?run=1&code=struct%20SomeStruct%3C%27struct%3E%20{%0A%20%20%20%20a_string%3A%20%26%27struct%20str%2C%0A}%0A%0Afn%20main%28%29%20{%0A%20%20%20%20let%20x%20%3D%20box%20SomeStruct%20{%20a_string%3A%20%22A%20string%22%20}%3B%0A}