Added in_menu and not_in_menu methods to PageQuerySet

This is handy for building menus in templates.

Example:
```HTML+Django
<ul class="menu-items">
    {% for page in home_page.get_children.live.in_menu %}
        <li><a href="{{ page.url }}">{{ page }}</a></li>
    {% endfor %}
</ul>
```

This pulls in my wagtailcore-cleanup branch (#330)