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, @alext - updated.

don't think YAGNI fits here. we need all the features that the gem provides. we only have to pick between writing some presentation logic ourselves vs delegating it to the gem. we use active record, we don't use the whole of it, but we decide to use ORM and so we pick active record. similarly, with AM serialisers i was choosing the approach to use serialisers to encapsulate presentation logic and use its tests to give me confidence about working code.