Upgrade to Rails 3.2

Cools
-----

* Haml has finally been updated to >4, and the ensuing "undefined
  Sass::Plugin" error addressed
* ...which fixes the "sass is being separated from haml" deprecation
  warning
* jquery-rails no longer includes jquery-ui. Instead, there's
  jquery-ui-rails, which allows including only the three modules we
  actually use

Bummers
-------

* vendor/plugins is now deprecated. Huge and repetitive deprecation
  warnings are now the norm.
* ActiveSupport::Memoize has been deprecated. More deprecation warnings.
* Something in the Rails 3.2 stack seems to monkeypatch `hash` into my
  objects, so I renamed TableOfContents#hash to TableOfContents#to_h,
  which I like more anyhow.