Add an api for customising how verifying doubles are checked

This is a prerequisite for rspec/rspec-rails#1238
Thanks for changing this as I suggested.  I'm realizing that passing the `ref` to the block like this forces us to declare the various object reference types as `@public` and to document them (at least, we should if we are making `when_declaring_verifying_double` public).  Currently they are private.