Add `respond_to` and `respond_with` to controllers

- All of the routes that used to return a variable will now respond
  with json or html depending on the request.
- Some of the endpoints return more than one thing, but I wanted to
  make this change without breaking the current views.
Merge pull request #41 from meltar/respond_with_json_or_html

Add 'respond_to' and 'respond_with' to controllers