Allow user to trigger upload start by passing in an HTML5 file drop event

I wanted to use this plugin to handle the uploading part of a drag/drop file upload (like how Gmail attachments work) but the existing code would only begin upload directly from the file input.

I had to refactor slightly, but I tried to keep the core code very simple and not doing too much. This code just letsthe user trigger the upload to start by passing the file drop event itself. 

I've added a clear example in the example.html file that demonstrates how to use this.