Adding example using aggregate

This example shows how to use aggregate for the template fixtures in
`test/fixtures/books`

Added book.hbs for the layout
Added toc.hbs for the aggregate example
Added `compare_fn` option in the Gruntfile that's used to sort
each page by the book chapter defined in the yfm

Renamed the chapter files so they were out of order by title to show
how sorting on yfm works.