Add option to as_json to allow links to be omitted

I am using as_json to include nested resources and typically I don't want the links.

Therefore I added and options param to allow this to be specified and in a serializer i can go:

 ```
 def user
    UserSerializer.as_json(@model.user, {}, no_links: true)
 end
```
Not sure if this screws up something else, but all tests passing so far....