std::io::TempDir::new and new_in should return IoResult

[`std::io::TempDir`](http://doc.rust-lang.org/std/io/struct.TempDir.html) currently has associated functions 

``` rust
fn new_in(tmpdir: &Path, suffix: &str) -> Option<TempDir>;
fn new(suffix: &str) -> Option<TempDir>;
```

These should return `IoResult` rather than `Option`, so that the result can be used with `try!()`.
