Fix utf-8 reading in Example 5

Pushing `u8` characters into a `String` is guaranteed to fail with any
non-ascii chars.

Instead, we push into a `Vec<u8>` and then call `from_utf-8()`.

We're still reading the file wrong for every file that isn't utf-8
encoded, but it's still better than nothing.

Sorry about not committing from the tip, I have rust 0.12 here and can't compile it.

(also, disclaimer, I'm a Rust newbie, just having fun learning the language)