from_str has an impl for uint, not Option<uint>

See: http://doc.rust-lang.org/std/from_str/trait.FromStr.html
```
let input_num = from_str::<Option<uint>>("5");
```
```
<anon>:2:21: 2:45 error: failed to find an implementation of trait std::from_str::FromStr for core::option::Option<uint>
<anon>:2     let input_num = from_str::<Option<uint>>("5");
                             ^~~~~~~~~~~~~~~~~~~~~~~~
```