Add missing test dependences to Appraisals

Since as of commit 2748b750873faf587c78d7a71f6c725b0fd4252a, we no
longer install dependencies inside of the Rails app that is generated
and used to run all of the tests, we have to require all of the
dependencies that the app would install inside of the appropriate
Appraisals.

This was mostly straightforward except for some workarounds with the
turn gem:

* Rails 3.1 requires two versions of turn depending on which Ruby
  version you're using. On 1.9.2, it uses turn 0.9.2; after 1.9.2, it
  uses ~> 0.9.3. To accommodate this we have to have two versions of the
  Rails 3.1 appraisal which declare the different turn versions.
* Rails 3.1 also loads the turn gem even if, in the Gemfile for the app,
  turn is declared with `require: false`. This causes a problem while
  running our tests because turn actually requires minitest/autorun,
  which adds a hook so when Ruby exits, Minitest tests are run. Because
  we're already using RSpec, Minitest will try to re-run the `rspec`
  command we ran within a Minitest environment. This will fail since we
  are using RSpec-specific command line options to run the tests.
  Unfortunately there's no way to shut off minitest/autorun after it's
  been required, so we have to monkey-patch Minitest's #run method so
it's a no-op.
Move dev dependencies to Gemfile

Currently some dev dependencies are in the Gemfile, and some are in the
gemspec. The dependencies in the Gemfile are there because they are
using Bundler-specific options, so there is a legitimate reason for them
to be there, however, it's confusing to have gems in two places. The
common denominator is the Gemfile, so let's put them there instead.
Extract examples in README to inline documentation
Generate docs using YARD
Publish new docs when releasing a new version