allow naming verifying doubles just like ordinary doubles

Below is just a proof-of-concept for feedback before I pull up my sleeves.

The parameter validation is pretty much handled by TestDouble, while the name doesn't seem "verifiable" without setting up tests for failures (which makes a weird/roundabout way for testing this) - so there are a few ways of going about specs for this, and none of them seem ideal to me.

Issues/questions:
1) Personally, since verifying doubles depend completely on TestDouble, I'd check if they call the superclass with the right parameters - as opposed to duplicating specs for all *_doubles (but I'm guessing it's better to duplicate specs explicitly because of the various classes)
2) not yet implemented for other classes (class_double, object_double, instance_spy, object_spy, class_spy - anything else?) 
3) test is in `verifying_doubles/instance_double_with_class_loaded_spec.rb` - which seems like the wrong place (unless the test cases are duplicated in remaining spec files)
4) missing tests in `verifying_doubles/construction_spec.rb` (although I can only really test for invalid parameters - which would be pretty much testing TestDouble initializer anyway)
5) docs not updated (for all mentioned classes)
IMO, the user of `let`, `before`, and `subject` here is overkill and something much simpler works just as well while being less confusing. (The `error_message.replace(e.message)` in particular is a code smell, I think).

``` ruby
context "with an expectation error" do
  def error_message_for(*double_args)
    instance_double(*double_args).defined_instance_method
  rescue MockExpectationError => e
    e.message
  else
    raise "should have failed but did not"
  end

  context "when a custom name is given" do
    it 'uses the custom name in error messages' do
      expect(error_message_for("LoadedClass", "foo")).to include('Double "foo"')
    end
  end

  context "when no custom name is given" do
    it 'uses the custom name in error messages' do
      expect(error_message_for("LoadedClass")).to include('Double "LoadedClass (instance)"')
    end
  end
end
```