Add strict mode to Mocker

[phy1729]

It would be nice to be able to assert that all mocks are used to catch bugs where a code path was inadvertently disabled. This PR adds a strict mode to Mocker so tests that fail to use all responses of all _Matchers will raise an AssertionError.

I tried to follow style and add relevant tests and stubs, but let me know if I missed anything.

[jamielennox]

Exhausted is simply another way of doing count > 0. Initially there had been the intent to provide this kind of helper function, but all the projects i've worked on have ended up writing them their own way as a method instead of on the mocker.