Added paged collections responses, using stories as the first one.

Added PagedCollection model and PagedCollectionRepresenter module to handle HAL responses for list of results (e.g. index actions).

Chose kaminari for pagination, but not set in stone

Made some changes to the root doc, including new 'stories' method and showing as templated.

Not sure we will need it, so perhaps premature over-engineering, but I added an abstract super class for models.

Also did some cleanup to get rid of other serializers and piece related classes.