Add route for CSV of tag counts. Refs #468

There were no unit tests in TagController, and it needs a rewrite.
So I didn't add any unit tests for my method :).
Upgrade underscore.js
Add options to create_tag(), with beforeChange() and callback().

These two callbacks can be used to differentiate between updates from
the server and updates from the user. We need a more general way of
doing this ... at which point we can revisit this
Add OnDemandTree.get_root() method
Allow passing an Object to TagStore.create_tag()
Add Backbone to DocumentSet.show bundle config
Create TagList Backbone view. Refs #468
Create TagStoreProxy, which brings TagStore to Backbone. Refs #468.
Create TagDialogController, which creates a TagList view. Refs #468
Add TagList dialog to the page, with an "organize..." link. Fixes #468
Remove little edit buttons. Refs #468