Add a method to LimitReader to return the limit

This is useful in contexts like this:

let size = rdr.read_be_i32() as uint;
let mut limit = LimitReader::new(rdr.by_ref(), size);
let thing = read_a_thing(&mut limit);
assert!(limit.limit() == 0);
auto merge of #12299 : sfackler/rust/limit-return, r=alexcrichton

This is useful in contexts like this:

```rust
let size = rdr.read_be_i32() as uint;
let mut limit = LimitReader::new(rdr.by_ref(), size);
let thing = read_a_thing(&mut limit);
assert!(limit.limit() == 0);
```