Content libraries paging

Changing this to load only the selected children is actually fairly straightforward... you can also modernize the code at the same time. The only problem is that in the future when we add sorting, it may be necessary to load all the children, sort them, then select the current page subset.

Code changed to support selective loading:
```python
def author_view(self, context):
        """
        Renders the Studio preview view with HTML appropriate for Studio.
        """
        fragment = Fragment()
        contents = []

        paging = context.get('paging', None)
        item_start = 0
        children_to_show = self.children  # <--- This is the main change - self.children won't load the XBlocks
        children_count = len(self.children)

        # TODO: sort children

        if paging:
            page_number = paging.get('page_number', 0)
            raw_page_size = paging.get('page_size', None)
            page_size = raw_page_size if raw_page_size is not None else children_count
            item_start, item_end = page_size*page_number, page_size*(page_number+1)
            children_to_show = children_to_show[item_start:item_end]

        for child_key in children_to_show:
            child = self.runtime.get_block(child_key)
            child_view_name = LibraryModule.get_preview_view_name(child)
            rendered_child = self.runtime.render_child(child, child_view_name, context)
            fragment.add_frag_resources(rendered_child)

            contents.append({
                'id': unicode(child_key),
                'content': rendered_child.content
            })

        fragment.add_content(
            self.system.render_template("studio_render_paged_children_view.html", {  # pylint: disable=E1101
                'items': contents,
                'xblock_context': context,
                'can_add': True,
                'can_reorder': False,
                'first_displayed': item_start,
                'total_children': children_count,
                'displayed_children': len(children_to_show)
            }
        ))
        return fragment
```