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: 
A little nitpicky but this would be nice with line numbers since you call one out. Playpen has them but [rustbyexample](http://rustbyexample.com/hello.html) doesn't embed them...not really in the scope of guide writing though...