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.
Simplify installation instructions

Also, shoulda-matchers needs to be required in rails_helper under RSpec
3.x instead of spec_helper.
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. So the
common denominator is to have them in the Gemfile.
Extract examples in README to inline documentation
Generate docs using YARD
Publish new docs when releasing a new version
Remove link to documentation on github.io for now
Fix method links in README to work on GH and YARD
Rename yard.watchr to docs.watchr

I keep wanting to write this, so let's just change it.
Use proper enumerable method

Instead of using `select` and a not to get all the false matchers
we are now using `reject` to return them.
Allow multiple StrongParametersMatchers to exist at once.

The provided RSpec example looks a bit contrived, but it corresponds to
this perfectly reasonable Minitest test case that was not working:

  class UserControllerTest < ActionController::TestCase
    should permit(:name).for(:create)
    should_not permit(:admin).for(:create)
  end

This instantiates two matchers at class load time, which resulted in the
second one overriding the double collection of the first one. Then when
the tests are executed, the first matcher would fail, because its double
is not listening to ActionController::Parameters#permit -- only the
second matcher's double collection gets activated.

This is fixed in Doublespeak by only creating one double collection per
class.
Add `validate_uniqueness_of_(:foo).allow_blank`

Complements [`validates_uniqueness_of(:foo, allow_blank:
true)`](http://api.rubyonrails.org/classes/ActiveRecord/Validations/ClassMethods.html#method-i-validates_uniqueness_of).
Removed `ActiveModel` 3.1 check

> 3.1 is end of life, I don't think we need to support it here.
- @sgrif
Extracted `value_for_new_record`

> Can you extract this into a private method?
- @mike-burns
Fixed version in `NEWS.md`

> Can you drop the version number and put this in HEAD, instead? This
> will likely go in 2.7.0, but we're not sure what version number it
> will be until we decide to make the release.

- @mcmire
`define_model_with_allow_blank` returns a `Class`

returning a `Class` and calling `new` is more explicit
Hound Fixes