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.
Relish is very picky about line breaks in rendering.  Have you pushed this to the staging environment to confirm it looks good?  (Run `rake relish_staging` to push it).