Fire an event (ajax:remotipartComplete) when the ajax upload completes

This change will fire an event (`ajax:remotipartComplete`) when the upload completes (i.e. fails or succeeds).

I was having trouble hooking in to remotipart from my javascript. Not sure if it was a Rails 4 thing, or just my wanting to render plain ole JSON, not create.js.erb. 

Anyhoo, I'm going to use this change, as now I can do the following:

`app/assets/javascripts/application.js`

    $(form).on("ajax:remotipartComplete", function(e, data){
       console.log(e, data)
       // or
       // object_json = JSON.parse(data.responseText);
    });