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.
This is missing a word, I think, maybe should be "...to easily create null object doubles..."?