Created a base class that doesn't return an HttpResponse object

I've created a new class called RawAjaxFileUploader which just performs the upload and returns the response dictionary to the client code. This allows extra server-side processing (such as saving the path the uploaded file in the database).

AjaxFileUploader is a subclass of this class which behaves exactly as it used to, returning the response serialised and wrapped in an HttpResponse object.