Add an api for customising how verifying doubles are checked

This is a prerequisite for rspec/rspec-rails#1238
This spec (and the one below) has the potential for being a false positive since the expectation is in the `when_declaring_verifying_double` block if the block is never called.   I think it would be better to use a yield matcher:

``` ruby
expect { |probe|
  RSpec::Mocks.configuration.when_declaring_verifying_double(&probe)
  class_double CustomModule
}.to yield_with_args(CustomModule)
```