Create the API for viewing a single cookbook.

:fork_and_knife: :fried_shrimp:

This is at a point where I feel good about it getting reviewed. This PR creates the API end-point for getting a single cookbook. For example `GET /api/v1/cookbooks/redis`. It does not handle ratings yet. It returns `null` for `average_rating` at this time. If the specified cookbook does not exist, a 404 is returned. Here are the current docs for this end-point: http://docs.opscode.com/api_cookbooks_site.html#id1

Left todo:

- [x] test with knife
- [x] refactor the controller and view logic