Added ability to remove spies from object methods

[wsb9]

Feature is useful when you need to spy on system/browser APIs and restore it after test is done.

Test case for new functionality is included.

[keithamus]

Hey @wsb9 thanks for the PR!

We've actually got a issue filed here: #38 in which we're trying to work out a design for how to do spy restoration.

I think I'd really like it to be outside of the spy object, so that we don't unnecessarily pollute the spied Functions namespace.

I really appreciate the work you've put into this, and I hope this doesn't put you off of contributing further -but I think for now I'm going to close this. I'd really like it if you gave some of your insight into #38 and we came up with a really nice design for this - and then we can look at making PRs.

[wsb9]

Oops, didn't noticed such thread, will take a look :)