Comparable Klein and KleinResource

For unit testing my Klein-using application, it seems it would be very useful to be able to compare KleinResource instances.  For example, here is how I was previously testing the anonymous case of my `IRealm` implementation:

```
    def assertIsAnonymousResource(self, resource):
        """
        Assert that C{resource} is the object that represents the v2
        anonymous avatar.
        """
        # TODO: It's hard to make a good assertion in this test.  What
        # exactly should we assert about it?  The idea here is that we
        # at least know that the anonymous avatar supports the
        # "versions" endpoint so maybe if we get a 200 response back
        # from an attempt to render that, we have the right object.
        request = dummyRequest(b"GET", b"/versions", Headers(), b"")
        render(resource, request)
        self.assertEqual(OK, request._code)
```

With equality on KleinResource, I can test it like this instead:

```
    def assertIsAnonymousResource(self, resource):
        """                                                                                                                                            
        Assert that C{resource} is the object that represents the v2 anonymous                                                                         
        avatar.                                                                                                                                        
        """
        self.assertEqual(
            _AnonymousUserV2(versionService=self.versionService).app.resource(),
            resource)
```

I much prefer the latter.

A potential downside is that Klein is full of magic and the semantics of equality are therefore somewhat magical.  I suppose it's possible people will be surprised by the behavior this branch implements.  Documenting the behavior might mitigate this somewhat but I didn't notice a `doc` directory in the repository and didn't feel like starting one.