Add an api for customising how verifying doubles are checked

This is a prerequisite for rspec/rspec-rails#1238
I like `target` a lot better than `const`.  I was going to suggest we take it a step further and call it `verification_target` but I think that the actual double is the target (the thing being verified) and now I wonder if `verification_source` is a better name (as the objects returned by these methods provide the source for the interface we verify against).  Thoughts?