Add RingBuf examples

This adds examples for get(), get_mut(), swap(), iter() and mut_iter()
This could be replaced by an assertion about what the iterator produces. Something like this would work:

```rust
    assert_eq!(buf.iter().collect::<Vec<&int>>().as_slice(), &[&3, &4, &5]);
```

It is unfortunately a little more complicated-looking than I'd like, but it does assert the behavior instead of merely providing a comment. And you can use the same thing in the example for `mut_iter()`.

FWIW this would be a little simpler with `move_iter()` but apparently `RingBuf` doesn't implement that?