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.
@alext - i wasn't able to get `as_json` working with the `:only` option [as documented](http://api.rubyonrails.org/classes/ActiveModel/Serializers/JSON.html#method-i-as_json). even if we get `as_json` working, we'll have to write some code to conditionally include an errors hash into the resultant JSON.

using this gem:
* keeps the controller clean by keeping the rendering logic in serialiser classes
* serialisers are more succinct compared to presenters, where we have to write some bioler-plate code to conditionally include/exclude attributes. instead here we only write some config-like code.
* presentation logic is already tested within the gem, and we don't need to write explicit presenter tests to check for presence of attributes, error hash etc.
* this gem comes from the [`rails-api`](https://github.com/rails-api) team, and they keep this gem fairly updated.

for me, the most compelling reasons are succinctness and re-use of tested code. hence, i'd suggest we use AM serialisers not only here, but in other places as well.