Restoring API controllers and routes.

- adds the `Answers` namespace to API controllers
- adds RESTful routes in config/routes.rb to expose these controllers
- fixes a few small errors in the Tag and Tagging API controllers

### QA

Try out these routes in your browser (or http client):

- `/api/v1/answers`
- `/api/v1/answers/1`
- `/api/v1/questions`
- `/api/v1/questions/1`
- `/api/v1/tags`
- `/api/v1/tags/1`
- `/api/v1/taggings`
- `/api/v1/taggings/1`

Routes that end with 1 (these are all `show` actions) should display json indicating a 404 not found (assuming there's not test data). `index` routes should display some json with an empty array with the model name as the key.

#### Bonus

Try this with test data in the db.