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.
@tekin - i've tried this. when i pass `only` to `as_json` it gets completely ignored and all attributes are returned in the response. i've spent time to find the root cause, and it seems many others have had issues getting this working.