Added PageSorter and PagesByDate

With this you can order lists of pages by their date like this:

```html
<ul>
    {{ range .Site.Indexes.groups.layout.PagesByDate true }}
    <li hugo-nav="{{ .RelPermalink}}"><a href="{{ .Permalink}}"> {{ .LinkTitle }} {{ .Date }}</a> </li>
    {{ end }}
</ul>
```
I feel kind of bad for adding another implementation of the Sorter interface but that's go is guess.

The other thing I feel bad about is the two different by functions but I think it's much better than checking sort order on every comparison.