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 think this method on the `Entry` class will give us every thing we need right now:

```ruby
  def as_json(options=nil)
    super(only: [:content_id, :title, :format, :base_path]).tap do |as_json_hash|
      as_json_hash[:errors] = errors if errors.present?
    end
  end
```