Add verifying double features from rspec-fire.

This is intended to be both API compatible with [rspec-fire](https://github.com/xaviershay/rspec-fire), and to completely obsolete it. It does not yet implement any of the suggested changes in #227, such as transferring nested constants by default or an alternate constant stubbing interface.

It adds the following behaviours:

```
verifying doubles
  class doubles
    when doubled class is not loaded
      allows any method to be stubbed
    when doubled class is loaded
      can transfer nested constants to the double
      can replace existing constants for the duration of the test
      only allows class methods that exist to be expected
      only allows class methods that exist to be stubbed
  instance doubles
    when doubled class is not loaded
      allows any instance method to be stubbed
    when doubled class is loaded
      only allows instance methods that exist to be expected
      only allows instance methods that exist to be stubbed
      checks the arity of stubbed methods
  when verify_constant_names config option is set
    prevents creation of instance doubles for unloaded constants
    prevents creation of class doubles for unloaded constants
```

It adds a perhaps unexpected method to the public API: `ArityMatcher.match!`. This seemed useful enough on its own to make public, but I would consider keeping it private if anyone exists.

### Suggested reading order

If you are not familiar with `rspec-fire`, start with `features/verifying_doubles/README.md`. `lib/rspec/mocks/example_methods.rb` is the entry point for the feature. Trace that through until you find `lib/rspec/mocks/verifying_proxy.rb` which contains the bulk of the logic.

### Questions for reviewers

* I'm not familiar with yardoc style. Does this conform?
* What is the preferred way to split features over the `README` and actual cucumber files?
* It is unfortunate that I need to use a parallel class hierarchy for the verifying counterparts. Is there a better way to do this?

@myronmarston @jonrowe

Fixes #227.