Fix the “clicking on anchor links puts you too far” bug for headers.

Look how easy this fix is:

```css
h1, h2, h3 {
    margin-top: -40px;
    padding-top: 60px;
}

h4, h5, h6 {
    margin-top: -50px;
    padding-top: 60px;
}
```

However, this might be hacky.  Test it: [Live version of the fix](http://getnikola.com/handbook-with-anchor-fix.html#installing-nikola) — I will be doing that through Sauce Labs for some browsers.  I will add a report card here.