Use separate CSS classes for the next and prev rellinks

[asmeurer]

This is as suggested at #101. This makes them easier to style separately, such
as making the previous link float left.

I personally am using a custom.css like

nav#rellinks {
    float: left;
    width: 100%;
}

.related.prev {
    float: left;
    text-align: left;
    width: 50%;
}

.related.next {
    float: right;
    text-align: right;
    width: 50%
}

nav#rellinks li+li:before {
    content: "";
}

Which makes it look like

[bitprophet]

Thanks, nice and simple. Figure I'll put this out with the next release, while we're stuck in this horrid "all releases are tertiary releases" nobody's gonna be put out by a purely additive new feature. Semver only goes so far :D

[asmeurer]

The Travis failure is some unrelated Python 2 thing by the way.

[asmeurer]

Can this be merged?