404.html: better content vertical alignment

Content vertical alignment using `display: table` for `<html>` and `display: table-cell` for `<body>`.
That way one can modify the content and don't worry about readjusting the margin.
> ```css
h1 {
	line-height: 0.9;
	margin-bottom: 0.3em;
}
```

![](https://cloud.githubusercontent.com/assets/1223565/3764123/e7b54dba-18ac-11e4-865b-de1ca911bcff.png)

@Roope  maybe something like:

```css
h1 {
    margin: 0 0 0.3em 0;   /* having it like this compresses better */                             
    line-height: 1.2;
}
```

![](https://cloud.githubusercontent.com/assets/1223565/3764135/1e257f8c-18ad-11e4-8c03-f6e1dee6819c.png)