Migrate `should` syntax specific specs to their own file.
Convert specs to RSpec 3.0.0.beta2 syntax with Transpec

This conversion is done by Transpec 1.9.3 with the following command:
    transpec -c "bundle install && bundle exec rspec"

* 295 conversions
    from: obj.should_receive(:message)
      to: expect(obj).to receive(:message)

* 128 conversions
    from: obj.stub(:message)
      to: allow(obj).to receive(:message)

* 65 conversions
    from: Klass.any_instance.should_receive(:message)
      to: expect_any_instance_of(Klass).to receive(:message)

* 50 conversions
    from: Klass.any_instance.stub(:message)
      to: allow_any_instance_of(Klass).to receive(:message)

* 28 conversions
    from: obj.stub_chain(:message1, :message2)
      to: allow(obj).to receive_message_chain(:message1, :message2)

* 24 conversions
    from: obj.should_not_receive(:message)
      to: expect(obj).not_to receive(:message)

* 7 conversions
    from: Klass.any_instance.should_not_receive(:message)
      to: expect_any_instance_of(Klass).not_to receive(:message)

* 7 conversions
    from: obj.stub(:message => value)
      to: allow(obj).to receive_messages(:message => value)

* 3 conversions
    from: Klass.any_instance.stub_chain(:message1, :message2)
      to: allow_any_instance_of(Klass).to receive_message_chain(:message1, :message2)

* 2 conversions
    from: Klass.any_instance.stub(:message => value)
      to: allow_any_instance_of(Klass).to receive_messages(:message => value)

* 1 conversion
    from: obj.stub(:message).and_return
      to: obj.stub(:message)
Fix any_instance specs after transpec has updated them.

- Some specs only apply to the old :should syntax.
- Some specs were testing invalid things that transpec
  removed.
Fix a use of `stub` that was missed by transpec.
Migrate some more specs that are `should`-specific.
Disable :should syntax by default.

Fixes #600.
Remove old comment.
Update doc strings so they don't mention legacy APIs.
Add some basic sanity check specs for `should` syntax.

Since we've migrated to the expect syntax we lost a lot
of test coverage for the should syntax.  For the most part
we don't need everything covered for both because it's the
same underlying mechanisms but some sanity tests are nice.