Add example for from_byte() documenation

I'm working on adding examples to the API documentation. Should future pull requests include examples for more than one function? Or is this about the right size for a pull request?
Use these in qualified form, for clarity:

    ```rust
    use std::str;

    let string = str::from_byte(66u8);

    assert_eq!(string.as_slice(), "B");
    ```