AutoNSubstitute

Integration of NSubstitute with AutoFixture.

I didn't reuse the code of the other pull request as I found it easier to start from scratch, first writing the unit tests and than writing the code.

Obviously, the code is heavily influenced by the existing integrations - namely AutoFakeItEasy - but my starting point always was the required interface / type of the AutoFixture API.
The result is code that is similar in functionality to that of the other integrations but still respects the unique features of NSubstitute.
One example is the fact that there is no `Fake<T>`, `Mock<T>` or similar type which removes the need for a relay.