RingBuf's get() is broken

```
let mut rb = RingBuf::new();
rb.push(111u);

assert!(rb[0]==111);
rb.get(0).expect("before pop");

rb.pop_front().unwrap();

rb.push(222);

assert!(rb[0]==222);
rb.get(1).expect("must fail");
rb.get(0).expect("after pop");
```

This will panic with "after pop", same for get_mut.

rustc --version gives me only "rustc 0.13.0-nightly". (Installed Nov 10)
