Add fn skip(n: uint) with a default implementation to Reader

Reader should have an extra fn that allows skipping a number of bytes. The default implementation could look something like:

```
fn skip(n: uint) { let _ = self.read(Vec::from_elem(n, 0u8); }
```

However, BufReader could implement the fn by simply updating the internal cursor. This would allow more efficient skipping w/o having to require Seek.
