AJAX uploading plus a fallback html form

Added the following ajax file upload plugin:
http://valums-file-uploader.github.io/file-uploader/

Made the following overrides in the plugin configuration:
* Replaced the plugin's template to use boostrap classes.
* Disabled the cancel handler and spinner to reduce clutter.
* Disabled the drag and drop handler as they did not appear to be
  working in any browser tested (may revisit soon).
* I ignored the plugin's included stylesheet, opting to recreate
  the essential styles for conditional hiding.

Reworked the upload handler to respond in two formats:

* /upload.json - Used by the AJAX uploader plugin, responds with JSON.
* /upload - Fallback mode for a vanilla html form. Responds with a
  redirect and flash messages according to state. Provides a decent
  experience for browsers that don't support JS for whatever reason.