Add ability to remove a spy

I added a stop_spying_on() analog to spy_on()

The motivation for this was that we needed to remove spies to avoid problems with teardown in our specs. The problem with relying on the CDRSpyInfo afterEach is that it gets called after some of our own. I also think it adds some nice symmetry to the tests in that you clean up after yourself in the same file.

Thanks,
J.J.