Backbone model/view for asset list.

Summary:
1. Converts the asset list to backbone model and view.
2. Moves the rest of the JavaScript in assets.js to the HTML page so that the functions do not have to be global and the "Upload File" action can access the Backbone view.
3. Fixes some rendering bugs with the file upload dialog (especially for 2nd and subsequent uploads on the page with no page reloads in-between). There was a method in base.js (markAsLoaded) that was never being called because the condition was never met, so I deleted that as well.
4. Deletes assets.py assets_to_json_dict method, which was not being used.
5. The beginning of a restful interface for updating assets-- right now only the Delete is implemented.

Acceptance tests are passing.

@dmitchell @singingwolfboy Please review.

@jkarni I can talk over the changes related to the asset upload dialog with you. I noticed that in startUpload, files.length was 0 for any upload after the 1st (until the page was refreshed). Therefore I changed how we get the filename so that we can properly configure the dialog each time an upload is done.