Add SetupFakeDbSet(), Add() and AddRange() extension methods

I've added a `SetupFakeDbSet()` method to make setting up an `IDbSet` easy in a unit test.  It's fluent so it can be used like this:

```` csharp
entities.SetupFakeDbSet(x => x.Books).Add(book);
````

It can also be used like this with the new `Add()` and `AddRange()` extension methods:

```` csharp
entities.SetupFakeDbSet(x => x.Books).Add(book1, book2);
entities.SetupFakeDbSet(x => x.Books).AddRange(books);
````

I would have raised an issue first, but you seem to have issues switched off on this repro.