useFakeXhr js spec support helper
extract ChapterPagesCollection from Chapter model

extend SubsetCollection to mix in orderedCollection instead of mixing
it into SubsetCollection itself.
do not save order of empty collection

refs #90

rails does not like when ids is empty array.
do not save order on receive

refs #90

update is hook is called anyway afterwards. prevent duplicate order
request which might related to MySQL dead lock errors.
consolidate positions in ordered collection on remove

removing an item at the top and adding a new item caused duplicate
position attributes.
Merge pull request #104 from tf/sorting

Improve Sorting Chapters and Pages