Post Upload Processing

Added flexible messaging with sensible defaults for further processing after file upload.

After the file upload, there may be a longer running server side process that does something with that file. For example: upload it to S3.

This update makes it so that it can show alternate messages in that case. Here's an example of how you would setup the view:

```
var uploadManager = new Backbone.UploadManager({
  uploadUrl: '/assignments/add_attachments_async',
  templates: {
    main: 'upload-manager.main.html',
    file: 'upload-manager.file.html'
  },
  doneMsg: 'Attached',
  processUploadMsg: 'Attaching...<img src="/media/images/loading_orange.gif">'
});
```