Categories api

Categories api with rest json
Seed data from http://reallygoodemails.com/

<code>Run <b>rake db:seed</b> to save it in db</code>
Still not sure why they were removed from Rails but I really like the pattern from using [Responders](https://github.com/plataformatec/responders) like this:

```
def update
  respond_with @category.update(category_params)
end
```

Then all the custom responses are all encapsulated with a common pattern. Any thoughts on implementing something like this?