This renames the "sections" list to "items". In practice, this list has
contained more than just "sections" since parts were added. Also, the
rest of the code consistently uses the term "items", since the values it
contains are called `BookItem`s. Finally, the naming has always been a
little confusing to me.
This is a very disruptive change, and I'm not doing it lightly. However,
since there are a number of other API changes going into 0.5, I think
now is an ok time to change this.
