Add an api for customising how verifying doubles are checked

This is a prerequisite for rspec/rspec-rails#1238
I don't consider the string form to be the primary use case.  Nor do I consider the class reference form to the primary use case.  It depends on your environment.  If the class is always loaded (such as is the case in rspec-core's specs), then passing the string name is inferior because if you typo it (or the class name changes) it will never get verified but you probably won't notice.  OTOH, if the class is a heavyweight dependency and you're able to run your specs w/o it loaded, that's very clearly a win for the string form.

Either way, the form that is used here is orthogonal to what is being tested.