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.
strange. i'm able to get that working now and can't replicate the [issue i was facing](http://stackoverflow.com/questions/9105755/rails-as-json-only-rendering-not-working). i spent considerable amount of time the other day trying to get `as_json` working as expected. :confused:

i agree that we have simple requirements for our JSON representations as of now, but in the future if it gets complicated we won't re-write our presenters so that we can introduce this gem. imagine the `content-store` presenters re-written using this, we'd save on writing/maintaining some of [these tests](https://github.com/alphagov/content-store/blob/master/spec/presenters/private_content_item_presenter_spec.rb). even if i switch to overriding `as_json` on the model, there will be some unit tests added.

also, it is not a case where we are introducing a gem and only using a subset of its features which we will need, nor is the gem big in size; which would be a good reason to defer the use of it. however, i'll go with the consensus as of now.