Add HighChart#as_json method

When using a HighChart object for returning JSON data in a rails controller it
is often appropiot to pass an object into the render method as a hash to be
converted into JSON.

    render json: @chart

The render method will process the hash offered by the json: using it's 
as_json method if it exists.

http://stackoverflow.com/a/2574900/227176 explains this better.