Entry resource CRUD

www.agileplannerapp.com/boards/173808/cards/7897

As a search and browse developer
I want to be able to discover the `content_ids` of topics
so that I can tag content to those topics

includes and `Entry` model, and controller actions to be able to submit entries, and retrieve entries belonging to a particular format.

i'll work on updating README and creating a `/doc` with input output examples if this looks good.
I like @alext's method better than mine.

I do think `active_model_serializers` is a nice gem and has good principals behind it. On the other hand, I think deciding whether we use it or not should be something we consider within the wider context of the Publishing 2.0 work. Ideally, both the content store and the content register will be consistent in the way it does things. Given that in both cases, we have a fairly simply requirements for the JSON representation of our models and that these are not likely to get significantly more complicated in the future, I would say we would be better served to defer using the gem until we know there is a strong case for it.