Adds spying methods to the RSpec Mocks DSL

Specifically adds:

* spy - responds and spies all methods
* instance_spy - responds and spies all methods to which a specified
  class responds.
* object_spy - responds and spies all methods to which a specific
  instance responds
* class_spy - responds and spies all class methods to which a specific
  class responds

I added test coverage for the expected behaviours and added some YARD
docs to the new methods.
Actually, nevermind about that -- I just realized this is the README, which isn't published to relish.  The suggestion applies when you update the narrative of `spies.feature`, though.

I also think it'd be good to for this to clarify what a `spy` is, and to mention that there are `instance_spy`, `class_spy` and `object_spy` versions.  Maybe something like this?

```
Verifies the given object received the expected message during the course of
the test. For a message to be verified, the given object must be setup to spy
on it, either by having it explicitly stubbed or by being a null object double
(e.g. `double(...).as_null_object`). Convenience methods are provided to easily
null object doubles for this purpose:

spy("invitation") # => same as `double("invitiation").as_null_object`
instance_spy("Invitation") # => same as `instance_double("Invitiation").as_null_object`
class_spy("Invitation") # => same as `class_double("Invitiation").as_null_object`
object_spy("Invitation") # => same as `object_double("Invitiation").as_null_object`
```