first pass of multi-uploader

I made a first pass at adding a multi-uploader functionality (#234). The front end uses https://github.com/blueimp/jQuery-File-Upload/ (the files are from release 9.5.7), the back-end is pretty simplistic. The workflow is as follows:

1. Visit ``admin/images/multiple/add/`` (no button yet)
2. Select files to upload and it begins automatically; I believe drag and drop works, as well but it isn't so clear what files are actually uploaded
3. An inline-like form gets added to the page, where the title can get fixed (defaults to the filename) and tags added
4. The changes can be saved or the image can be deleted ("upload cancelled"), both of which close the form

I never manipulate the file directly in the view, it only assigns the file to the model instance. I think because of this, the uploader can handle whatever file storage the model can (i.e., S3 buckets).

This was a first pass, and I'm very happy to make changes or pass it along to someone else to take over.