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.
"...in the future if..." is the [enemy](http://c2.com/cgi/wiki?YouArentGonnaNeedIt)! ;p. We don't even have any presenters here to rewrite yet, just a single custom `as_json` method. I spent a chunk of time unwriting some complex and unnecessary code on Thursday that had unwittingly introduced a bug and was written for some imagined future that never came to pass. It's almost always better to defer a decision like this if you don't know for sure instead of making it based on an "if".