Document how to extend server lifetime.

The example on the front page never prints "On 3000" because the lifetime of the `Listening` returned from `http().unwrap()` expires within its own statement and then blocks the program. The solution to this is to store this value in an unused variable so that its lifetime is extended until the end of the block.

For example:

``` rust
let _listener = Iron::new(hello_world).http("localhost:3000").unwrap();
```

The leading `_` suppresses unused variable warnings (but note that it cannot just be called `_` since that has no lifetime).

It would also be good to add a note to the `Iron.http` and `Iron.https` methods where they discuss the guard to explain how to prevent it blocking.