Moved callhistory classes to own namespace
Moved other non-interesting-to-users classes to FakeThat.Extras, so that `using FakeThat` does not pollute the global namespace too much.
Turned CallHistory classes into IEnumerable<TCall>s, replacing the Calls property.
Updated readme and some docs to new API
Better docs.
Removed extension method experiment and unused Fake base class.
Better docs